I mean, I was a kid ump - but I was good
Some LL's in the area will pay a kid 15 or 20 bucks to go do the bases. I thought it was good for the league if the Plate Ump didn't just ignore the kid all game but would work with him, talk in between innings, etc. I doubt they will learn much unless they get that experience. Doing coach-pitched doesn't provide game management skills, and thats the last thing that an umpire should learn anyway. If developing umpires is important to you or the league, then you should meet with them preseason as well to talk about curling in, being in the saddle, confidence, timing.
Then have a mid way through the season meeting as well with them. I was talking with a fellow umpire who is in a similar position to you, and he meets with the kid umps twice a season, just to reinforce these things (teenagers are forgetful
)