ChildBuilder methods should take IntoIterator, rather than a slice type

It's generally better to use an iterator type instead of forcing someone to use a `slice`. Since it's not always guaranteed that the children entities will be in an array-like container.

```suggestion
fn replace_children(&mut self, children: impl IntoIterator) -> &mut Self {
```

_Originally posted by @NathanSWard in https://github.com/bevyengine/bevy/pull/6035#discussion_r981546329_