Depends on their ages, but as a general comment shorter rods are much easier and less tiring than longer rods for young beginning casters. One option is TFO's Bug Launcher series.
Another general statement is that a 5 wt seems to be a pretty good general purpose size for most people to start with. For a beginning youth, I'd put a 6 wt line on it.
Fly Fishers International certified casting instructor