Fold Childrens (playground: https://stackblitz.com/edit/js-rskhjq)

dataset.json

{
  "childrens": [
    {
      "name": "foo_lvl_1",
      "childrens": [
        {
          "name": "foo1_lvl_2",
          "value": 13
        },
        {
          "name": "foo2_lvl_2",
          "childrens": [
            {
              "name": "foo1_lvl_3",
              "value": 10
            },
            {
              "name": "foo2_lvl_3",
              "value": 10
            }
          ]
        },
        {
          "name": "foo3_lvl_2",
          "childrens": [
            {
              "name": "foo1_lvl_3",
              "value": 10
            },
            {
              "name": "foo2_lvl_3",
              "value": 10
            },
            {
              "name": "foo2_lvl_3",
              "value": 10
            }
          ]
        }
      ]
    },
    {
      "name": "bar_lvl_1",
      "childrens": [
        {
          "name": "bar1_lvl_2",
          "childrens": [
            {
              "name": "foo1_lvl_3",
              "childrens": [
                {
                  "name": "foo1_lvl_4",
                  "value": 10
                },
                {
                  "name": "foo2_lvl_4",
                  "value": 10
                }
              ]
            },
            {
              "name": "foo1_lvl_3",
              "childrens": [
                {
                  "name": "foo1_lvl_4",
                  "value": 10
                },
                {
                  "name": "foo2_lvl_4",
                  "value": 10
                }
              ]
            },
            {
              "name": "foo3_lvl_2",
              "childrens": [
                {
                  "name": "foo1_lvl_3",
                  "value": 10
                },
                {
                  "name": "foo2_lvl_3",
                  "value": 10
                }
              ]
            }
          ]
        },
        {
          "name": "bar2_lvl_2",
          "childrens": [
            {
              "name": "bar1_lvl_3",
              "childrens": [
                {
                  "name": "bar1_lvl_4",
                  "value": 10
                },
                {
                  "name": "bar2_lvl_4",
                  "value": 10
                }
              ]
            },
            {
              "name": "bar1_lvl_3",
              "childrens": [
                {
                  "name": "bar1_lvl_4",
                  "value": 10
                },
                {
                  "name": "bar2_lvl_4",
                  "value": 10
                }
              ]
            },
            {
              "name": "bar3_lvl_2",
              "childrens": [
                {
                  "name": "bar1_lvl_3",
                  "value": 10
                },
                {
                  "name": "bar2_lvl_3",
                  "value": 10
                }
              ]
            }
          ]
        },
        {
          "name": "bar3_lvl_2",
          "childrens": [
            {
              "name": "bar1_lvl_3",
              "childrens": [
                {
                  "name": "bar1_lvl_4",
                  "value": 10
                },
                {
                  "name": "bar2_lvl_4",
                  "value": 10
                }
              ]
            },
            {
              "name": "bar2_lvl_3",
              "childrens": [
                {
                  "name": "bar1_lvl_4",
                  "value": 10
                },
                {
                  "name": "bar2_lvl_4",
                  "value": 10
                }
              ]
            },
            {
              "name": "bar3_lvl_3",
              "childrens": [
                {
                  "name": "bar1_lvl_4",
                  "value": 10
                },
                {
                  "name": "bar2_lvl_4",
                  "value": 10
                }
              ]
            }
          ]
        }
      ]
    },
    {
      "name": "baz_lvl_1",
      "childrens": [
        {
          "name": "bar1_lvl_2",
          "childrens": [
            {
              "name": "foo1_lvl_3",
              "childrens": [
                {
                  "name": "foo1_lvl_4",
                  "value": 10
                },
                {
                  "name": "foo2_lvl_4",
                  "value": 10
                }
              ]
            },
            {
              "name": "foo1_lvl_3",
              "childrens": [
                {
                  "name": "foo1_lvl_4",
                  "value": 10
                },
                {
                  "name": "foo2_lvl_4",
                  "value": 10
                }
              ]
            },
            {
              "name": "foo3_lvl_2",
              "childrens": [
                {
                  "name": "foo1_lvl_3",
                  "value": 10
                },
                {
                  "name": "foo2_lvl_3",
                  "value": 10
                }
              ]
            }
          ]
        },
        {
          "name": "bar2_lvl_2",
          "childrens": [
            {
              "name": "bar1_lvl_3",
              "childrens": [
                {
                  "name": "bar1_lvl_4",
                  "value": 10
                },
                {
                  "name": "bar2_lvl_4",
                  "value": 10
                }
              ]
            },
            {
              "name": "bar1_lvl_3",
              "childrens": [
                {
                  "name": "bar1_lvl_4",
                  "value": 10
                },
                {
                  "name": "bar2_lvl_4",
                  "value": 10
                }
              ]
            },
            {
              "name": "bar3_lvl_2",
              "childrens": [
                {
                  "name": "bar1_lvl_3",
                  "value": 10
                },
                {
                  "name": "bar2_lvl_3",
                  "value": 10
                }
              ]
            }
          ]
        },
        {
          "name": "bar3_lvl_2",
          "childrens": [
            {
              "name": "bar1_lvl_3",
              "childrens": [
                {
                  "name": "bar1_lvl_4",
                  "value": 10
                },
                {
                  "name": "bar2_lvl_4",
                  "value": 10
                }
              ]
            },
            {
              "name": "bar2_lvl_3",
              "childrens": [
                {
                  "name": "bar1_lvl_4",
                  "value": 10
                },
                {
                  "name": "bar2_lvl_4",
                  "value": 10
                }
              ]
            },
            {
              "name": "bar3_lvl_3",
              "childrens": [
                {
                  "name": "bar1_lvl_4",
                  "value": 10
                },
                {
                  "name": "bar2_lvl_4",
                  "value": 10
                }
              ]
            }
          ]
        }
      ]
    }
  ]
}

foldChildrens.js

function foldChilndrens(tree, options, level = 0) {
  const opt = {
    children: 'children',
    value: 'value',
    maxLevel: 2,
    clearValues: false,
    ...options
  };

  const calcLeafWeight = childrn => childrn.reduce(
    (summ, chld) => (
      chld[opt.children]
      ? calcLeafWeight(chld[opt.children])
      : chld[opt.value]
    ) + summ, 0);

  if (
    !tree[opt.children]
    || tree[opt.children].length === 0
  ) return tree;

  if (level < opt.maxLevel) {
    // Delete value from tree;
    if (opt.clearValues) {
      let {[opt.value]: omit, ...rest} = tree;
    } else {
      let rest = tree
    }
    
    return {
      ...rest,
      [opt.children]: tree[opt.children].map(child => foldChilndrens(child, options, level + 1))
    }
  }

  // Delete children from tree;
  let {[opt.children]: omit, ...leaf} = tree;
  leaf.value = calcLeafWeight(omit);
  return leaf;
}

foldChilndrens(dataset, {
  children: 'childrens',
  maxLevel: 3
}),