add despawn_children

[PhaestusFox]

What problem does this solve or what need does it fill?

currently, despawn_descendents is a recursive way of removing all children from an entity without removing the entity itself,
I don't know what use case it would have but the face there is a difference between despawn and despawn recursive why not at this choice to this approach too

What solution would you like?

either despawn_children() as a method to despawn children but non-recursively
or despawn_descendants_recursive() to do what despawn_descendents() does and change it to be non-recursive

What alternative(s) have you considered?

could add a root_children() and root_child(index) methods to allow children to be made rood nodes would also make it easier to fix #5584