There's an excellent pinned topic under Invertebrates, Amphibians etc. According to this, you need at least a gallon per frog, preferably more, depends what you're keeping with them. There's a load more useful info here about temp, filters, substrates, decor, etc.
(I've just been reading up as my daughter's setting up a tank for some!)
Sounds very familiar - I've negotiated my daughter down from her initial request for turtles, snake, gecko, and axolotls (we just don't have space!!) finally agreeing on frogs. Hope you will post updates on yours!