I really like this..I have to explain things to people that they otherwise won't understand all the time, so adult or child I try to find an analogy they will understand.
I suppose one way I might explain it to my child...
You know how when we cook a cake, we put certain ingredients together in order to make it? Well our brain is kind of like that... in order for it to be the perfect cake, there are several different ingredients in there.
Now imagine we're making a cake... and we forget the milk. It'll probably still be a cake... but it will be a little different. It might not be as fluffy or as soft as it should be.
Sometimes people's brains are a bit like a cake that is missing milk - it still works, but it's not exactly how it's supposed to be. Sometimes this can mean people end up sad, or angry, or have trouble doing certain things or understanding things... just as the cake has trouble being nice and fluffy without milk.
Some kids might not get that, but similar explanations seem to work for my daughter. I've always explained things like that to her.