Haven't gotten really deep into ukemi, but it seems like if you made it a game like limbo, that would be pretty fun for kids. Hold a broom-stick pretty high at first, have them forward roll underneath it. Then lower it as they progress. Then start with it low and have them roll over it. Raise the bar as they progress. This could probably be done with other falls, but would work best with forward roll because you need a bit of momentum to clear the bar.
Another thing you could do is leap-frog, but that requires a certain amount of skill with a forward roll. In case you don't know, leap frog is where kids take turns getting on all fours and jumping over each other's backs. Just have the kids on all fours line up laterally relative to the kid behind them and change the jump to a forward roll. How To Play Leap Frog
Pick Up Sticks
This is for kids who are quite comfortable with forward roll. Place an escrima stick or bokken or whatever on the ground. Kids have to roll, pick it up while rolling, and come out of the roll to a standing position. Once they're comfortable with this, divide into teams and do a relay race. Kids have to roll, grab stick, hand it off. Kid who gets it has to toss it (gently) on the ground, roll and get it. I'm actually not familiar with how well kids can grasp the body mechanics of ukemi, so this may not be for your younger kids, but for teens who have been doing it for a while.
Learning to Fall
For kids to learn to fall in the first place, you might start with using an exercise ball. You guys might already be doing this. The idea is to roll over the ball. It might help with kids who are afraid of losing control when going to the ground (ease the transition from standing to rolling). I couldn't find video of a forward roll with an ukemi ball, but here's one showing how to sprawl on one