In our department, we use our Juniors as we can — primarily as a top-notch Rehab Team. They each get "Rehab Bags" and at fire scenes (out of the hot zone), they get busy setting up a bottle switch-out station, and they're prepared with tarps, water, hot/cold packs, mini-first-aid kits, garbage bags (to collect empties), energy bars, etc..
The Juniors decide what's in the bags (with supervision, of course) and we make sure they train and work as their own "independent" rehab team on scenes. This gives them buy-in to the department, makes them feel useful (when age prevents them from participating in actual emergencies), and lets them see the importance of rehab for our firefighters. It's a win-win situation!