I had an ich out break in a tank with two dwarf frogs in it, neither got it but I removed before treating because I read mechalite green (most common in ich treatments) is toxic to frogs. So if they can get it, would be hard to treat.
If the tank has an outbreak, and the frogs receive a scraping injury, they'll be prone to the parasite just as any other animal in the tank. Best treatment is by using salt and increased temp. Watch out for secondary bacterial infections.