Add remove_children and remove_related to EntityWorldMut and EntityCommands (#18835)

Fixes #18834.

`EntityWorldMut::remove_children` and `EntityCommands::remove_children`
were removed in the relationships overhaul (#17398) and never got
replaced.

I don't *think* this was intentional (the methods were never mentioned
in the PR or its comments), but I could've missed something.

crates/bevy_ecs/src/hierarchy.rs

@@ -293,6 +293,11 @@ impl<'w> EntityWorldMut<'w> {
         self.add_related::<ChildOf>(&[child])
     }
 
+    /// Removes the relationship between this entity and the given entities.
+    pub fn remove_children(&mut self, children: &[Entity]) -> &mut Self {
+        self.remove_related::<ChildOf>(children)
+    }
+
     /// Replaces all the related children with a new set of children.
     pub fn replace_children(&mut self, children: &[Entity]) -> &mut Self {
         self.replace_related::<ChildOf>(children)
@@ -375,6 +380,11 @@ impl<'a> EntityCommands<'a> {
         self.add_related::<ChildOf>(&[child])
     }
 
+    /// Removes the relationship between this entity and the given entities.
+    pub fn remove_children(&mut self, children: &[Entity]) -> &mut Self {
+        self.remove_related::<ChildOf>(children)
+    }
+
     /// Replaces the children on this entity with a new list of children.
     pub fn replace_children(&mut self, children: &[Entity]) -> &mut Self {
         self.replace_related::<ChildOf>(children)
@@ -646,6 +656,26 @@ mod tests {
         );
     }
 
+    #[test]
+    fn remove_children() {
+        let mut world = World::new();
+        let child1 = world.spawn_empty().id();
+        let child2 = world.spawn_empty().id();
+        let child3 = world.spawn_empty().id();
+        let child4 = world.spawn_empty().id();
+
+        let mut root = world.spawn_empty();
+        root.add_children(&[child1, child2, child3, child4]);
+        root.remove_children(&[child2, child3]);
+        let root = root.id();
+
+        let hierarchy = get_hierarchy(&world, root);
+        assert_eq!(
+            hierarchy,
+            Node::new_with(root, vec![Node::new(child1), Node::new(child4)])
+        );
+    }
+
     #[test]
     fn self_parenting_invalid() {
         let mut world = World::new();

crates/bevy_ecs/src/relationship/related_methods.rs

@@ -105,6 +105,23 @@ impl<'w> EntityWorldMut<'w> {
         self
     }
 
+    /// Removes the relation `R` between this entity and the given entities.
+    pub fn remove_related<R: Relationship>(&mut self, related: &[Entity]) -> &mut Self {
+        let id = self.id();
+        self.world_scope(|world| {
+            for related in related {
+                if world
+                    .get::<R>(*related)
+                    .is_some_and(|relationship| relationship.get() == id)
+                {
+                    world.entity_mut(*related).remove::<R>();
+                }
+            }
+        });
+
+        self
+    }
+
     /// Replaces all the related entities with a new set of entities.
     pub fn replace_related<R: Relationship>(&mut self, related: &[Entity]) -> &mut Self {
         type Collection<R> =
@@ -383,6 +400,15 @@ impl<'a> EntityCommands<'a> {
         self.add_related::<R>(&[entity])
     }
 
+    /// Removes the relation `R` between this entity and the given entities.
+    pub fn remove_related<R: Relationship>(&mut self, related: &[Entity]) -> &mut Self {
+        let related: Box<[Entity]> = related.into();
+
+        self.queue(move |mut entity: EntityWorldMut| {
+            entity.remove_related::<R>(&related);
+        })
+    }
+
     /// Replaces all the related entities with the given set of new related entities.
     pub fn replace_related<R: Relationship>(&mut self, related: &[Entity]) -> &mut Self {
         let related: Box<[Entity]> = related.into();