check-children.php

<?php

$ids = array_unique([1,2,3,4,5, ...]);

// relation() is HasMany

// check if all $ids belong to this parent model:
$parent_model->relation()->whereIn('id', $ids)->count() === count($ids)

// check if $ids match all of the children records:
$parent_model->relation()->count() === count($ids)
// if necessary check what's missing and where:
$ids = collect($ids);
$related_ids = $parent_model->relation()->pluck('id');
$missing_input = $related_ids->diff($ids);
$missing_in_db = $ids->diff($related_ids);