Hehe I was going to suggest my photo too Nic! Have a nice bright nappy on each of them then have only the nappy in colour & the rest of the photo B&W (I can help with that part if you want!).

Get A to look between his legs... So standing with his bum in the air & a big smile looking through his legs.

If you can manage it, get M asleep on her belly with her legs tucked under her. Again do it in B&W with the nappy coloured.

I think it would be best to have no top, so the nappy is the main feature with nothing distracting from it. I can just picture it, if only you lived closer.....