The acoustics at the ancient Epidaurus Theatre is pretty amazing. The kids should have a good time playing around making noise.
When you stand on the stone in front of the seats and say something with a normal voice, the theater amplifies your voice. It's really cool! There's no echo. Your voice just gets louder and you'll be able to hear that.
Last Bit of Advice
Try to visit this Theatre on a crappy day. That way you'll encounter less of a line to stand on the stone and project your voice out to the theatre.
The theater still puts on performances. If you're interested in seeing one, do some research far in advance of your trip.