Kids are kids. They're inquisitive, they like exploring and a device as accessible as the iPad, they're going to explore.
Don't discipline them for experimenting with a system like the iPad. You're the one being obtuse if you think that's going to help.
If they dropped it on the floor, or were generally mistreating it - then I agree, take it away. For now, I'd just laugh quietly and delete the pictures.
Here I was wondering if you were trying to figure out how to turn it off so you didn't get screen shots of um... certain types of adult materials :P