docs: Add more child actor docs

site/docs/02-fundamentals/03-actors.mdx

@@ -72,6 +72,23 @@ class ShootemUpPlayer extends ex.Actor {
 
 Custom actors make it easy to hook into the actor lifecycle and encapsulate the actor's state better than a basic actor.
 
+
+## Child Actors
+
+Actors can be nested with within another, and all children have their transform (position, rotation, scale, z) relative to their parent. This is useful for a few a number of use cases.
+
+```typescript
+const parent = new ex.Actor({...});
+
+const child = new ex.Actor({...});
+
+actor.addChild(child);
+```
+
+1. Paper doll assemblies - Moving a parent body actor and having child actor arms, legs, head, etc. pieces move.
+2. Area Trigger Sensors - Adding invisible child actor to listen to collisions
+3. Temporary attachment - Attaching a player to a moving platform
+
 ## Actor lifecycle
 
 An actor has a basic lifecycle that dictates how it is initialized, updated, and drawn. Once an actor is part of a

site/docs/10-physics/08-colliders.mdx

@@ -112,3 +112,27 @@ const player = new ex.Actor({
   collider: capsule
 });
 ```
+
+### Child Actor
+
+Another way of accomplishing complex collision logic is to add child actors with different colliders and handlers.
+
+This is can be useful for implementing area of effect collisions or sensors when objects get close to each other.
+
+For example:
+```typescript
+const player = new ex.Actor({...});
+
+const sensor = new ex.Actor({
+  radius: 100
+});
+
+player.addChild(sensor);
+
+sensor.on('collisionstart', (evt) => {
+  if (evt.other !== player) {
+    console.log('something is within 100 pixels of player');
+  }
+})
+
+````