Hi yellowbell,
Lots of people over on the pond forum use kiddie pools for growing extra water plants, little water gardens, etc. It would be perfectly fine for you to use this for the toads. If you don't have a little water pump in standing water, it can become stagnant though, and be a breeding ground for mosquitoes.
Can your son give his pool up? .....or you could buy another one for him. But I would add some water plants to the pool with the toads and rocks. They filter the water, so you wouldn't need a pump. Small watergardens are wonderful things. You could check out the pond forum here and look for the posts on barrel/tub gardens.
I'm so glad you noticed the little fella and helped him out!

Thanks, I'll check out the pond forum. That would be great to be able to have a little pond (and I do mean little) without having to get a bunch of equipment. I'd love to have more toads in the garden.

Hi again Yellowbell,
I have a couple 100 gallon stocktanks that I just set on the ground. I live in zone 5, so I have to bury the plants for the winter, but I think you wouldn't need to in your zone. Here's a pic of my stocktanks. It attracts green frogs, tree frogs and toads. I just love it!
The chicken wire around it was to keep the coons out! I don't use a pump in these, and the water stays clear because of the filtering that the plants do. It's sort of messy.........but the frogs like it that way!