if a fly was 6 inches above the water and it dropped 6 inches it would be on the water so the fish wouldn't have to jump out of the water to get it.....
Here is basic explanation for a 10 year old.
It's called momentum. When the fish lunges towards the fly it carries momentum forward. It cannot stop instantly at high velocity right at the water level.
So the fish will carry on "jumping out of water" as it is trying to catch the fly. Try catching a fly in your fist and notice how your hand moves past the fly even though it was in different location. You over compensate.
Clearly science isn't for everyone which is why some people resort to fairytales or "because magic" explanations instead.