I didn't do anything to make them breed, I guess they were just happy and healthy. It's a cool thing when they do, you get heaps of eggs like frogspawn and they hatch into a tadpole like thing and slowly grow legs. Only a few actually made it to adult size though, most died or they ate the small ones.
I did hear if you add salt to the water it can encourage them to breed but not sure if it's true.
As you got your daughter to do so much research first you probably know all this anyway.
Just out of curiosity why can't you sell them if they breed?