you shopuld be teaching them the change-up right now. that will do two things,
first, it will make them learn how to change speed because remember that your team is a training camp where you play against other kids. you have to look at it that way. you want to make kids feell their first breaking heart lost, their first dominance on the mound, the first time they get crushed, the first time they get bad calls, the first time they win a tight game, the first time someone swing and miss on something else than an overpowering fastball right down the middle, the first time they come in relief and become the hero of the day, the first walk-off hit. so they are prepared for the higher lkevels. one big error coaches in little leagues do is they are going for the W which is totally crazy when you're coaching 9 year olds.
secondly, having an arsenal of pitches is something funny. noboduy can deny it. throwing fastballs all the time is boring at some point and you want to bring up that trick pitch (change-up) that wuill give you an edge. remember the kids not to worry about speed diference in other words, especially at this age you should try not to tell them what's the deal about it. they will throw it exactly like the pitch they know but with a different grip and that is the best change-up you can get. dont make them slow theuir arm speed kif it doesn't work well because remember you're teaching your kids how to play baseball. a slower arm speed change-up at higher level is the favorite pitch of a good hitter.
some kind of palmball grip where they just put the ball in their palm and wrap their fingers around it.
or, an other good one would be to grip the ball 2-seam fastball, rotate the ball backward so that the fingers are now in the horseshoe not toucvhing anything, now take your ring and you middle finger and put them right in the middle. the pinky and the index fingers should rest just outside the horseshow seams or anywhere where they feel comfortable with. the thumb can rest under the ball. tell them to throw the ball using only the middle and ring fingers exactly like they would if it was the index and middle fingers. thats a good way to learn the circle change which is going to be a devastating pitch when they'll be older. see which one work the best for them and tell them to stick with it.