In my experiences with kids I've gone with, the *best* age seems to be about 10 years old. Now, having said that, my DD went she was 6 as well as my son and they LOVED it. But it seems that around age 10, they really *get* all the humor and aren't old enough yet to pretend to be too cool for it (even though teens love the show also, they have a tendency to try to hide it).
I think your 5 year old will really enjoy it and your 3 year old won't get most of it but will enjoy the clapping, singing, and general raucous atmosphere. The young ones especially like the washboard playing at the end.