[Feature] Extend load_related to direct Child[].

Description

Currently, the `load_related()` host function only works with `derivedFrom` fields. We could use the same function for loading childs in a more explicit way instead of looping and loading each one by one.

Are you aware of any blockers that must be resolved before implementing this feature? If so, which? Link to any relevant GitHub issues.

The PR #4434 must be merged before this is a valid request.

Some information to help us out

• Tick this box if you plan on implementing this feature yourself.
• I have searched the issue tracker to make sure this issue is not a duplicate.

まず、issue で前提条件として特定されている PR #4434 を確認し、次に host 関数 load_related と、現在の derivedFrom のみに基づく動作を追跡します。Child[] を直接読み込む意図されたセマンティクスを確認し、load_related が子ごとのループなしでその明示的な子の読み込み経路をサポートした時点で作業完了とします。