Really nice picture - very "Through the Looking Glass" and "Alice in Wonderland". However, it doesn't really make me wonder what is off screen. It is enjoyable and complete in itself. I like this picture a lot.
Such a pity that you dont really get the sense they are looking into an aquarium. It could be anything really. Nice expressions on the kids faces though. Moving round a few degrees to get more faces and less reflection might have worked better.
Good idea.. but if the exposure was better, we could see that aquarium.. maybe with a different shutter sync, you could have exposed the background before the flash fired and the exposure ended.. a potentially good photo.