A custom Sitecore JSS rendering contents resolver, which serializes items into a tree-like JSON structure. More information at https://blog.vitaliitylyk.com/jss-and-arbitrary-item-hierarchies/

DescendantsTreeRenderingContentsResolver.cs

using Newtonsoft.Json.Linq;
using Sitecore.Data.Items;
using Sitecore.LayoutService.Configuration;

namespace VitaliiTylyk.JavascriptServices.RenderingContentsResolvers
{
    /// <summary>
    /// Builds a tree-like structure of datasource item's descendants
    /// </summary>
    public class DescendantsTreeRenderingContentsResolver : Sitecore.LayoutService.ItemRendering.ContentsResolvers.RenderingContentsResolver
    {
        protected override JObject ProcessItem(Item item, IRenderingConfiguration renderingConfig)
        {
            var jObject = base.ProcessItem(item, renderingConfig);

            if (item.Children.Count == 0)
            {
                return jObject;
            }

            jObject["items"] = ProcessItems(item.Children, renderingConfig);

            return jObject;
        }
    }
}