Layers, parenting and collisions.

So I have a parent object on a different layer to it's child. They both have box colliders.

Here's what I'm trying to do with it:

Layers I'm using and how they are set up in the layer collision matrix:

RedA : hits BlueB

RedB : hits BlueA

I have two of the objects I talked about before. One using red layers and one using blue. A is on the parent object while B is on the child(Whos box collider is bigger and is a trigger). Ontriggerenter is on a script on the parent objects. When the two objects come near eachother ontriggerenter is called 3 times. The red object calls it on both blueA and blueB while the blue object only calls it on blueB (opposite of what I'd expect).

Is anyone able to tell me why this is or who to get around it? Any help would be appreciated.