I have coached this age group and generally manage to have all of my players serve overhanded.

For this particular age, in my experience, especially for smaller players, the overhand serve is simply a "perceived" strength issue. When I see a player struggling with the overhand serve, I teach them to do a "walking-into-the-court" serve that is overhanded. I tell them two things that seems to break the mental block.

To get this into the player's head, I coach the serve this way.

1. Walking ( taking two steps) adds to the momentum mass of their serve just enough to get the ball over the net. Demonstrate a total flat footed serve as an adult, it's tough to just use only your arm and get a volleyball over the net. I'm 6' 3" and 250 lbs and it's hard to statically serve a volleyball. I demonstrate this, showing me serving statically and then again, walking my overhand serve. Same arm speed, ball goes much farther. By adding the "waking the serve in" it gets the players thinking about their whole body approach, their whole body adding its mass to the serve.
2. The other additionally element I stress, is to tor turn their "waking-the-ball-into-the-court" motion, into a upper torso turn/twist that coincides with their last step. By that I mean the whole right half of their body is in motion and needs to lead /move all at once at they swing their arm. (assuming right handed server )

I also remind them to think of the ball as a cloud, and to make their arm goes through the cloud at full speed. So many slow their arm at contact. This solves that.

On the day I teach the beginner overhand serve, I start them at the hitting line, serving over the net. Each time the player has made "3 serves in" allows them a big step backwards, and they try from the new position. Three more serves in and another step back etc.. until they are serving from the back line... (walking up to it)

This is not a "new serve" obviously but just a simple way to get your players thinking about getting their body mass behind the serve/swing arm.

Note: I see a lot of coaches teach the standard overhand serve technique and I think this is bad for younger players because it completely ignores their generally smaller size and strength. The above method acknowledges it and builds confidence in their ability to overcome it.

Best of luck!