For me personally, whilst I know many people expect young players to have a "position", for U12's and U14's, I usually try and expect all 10 players to be able to do a little of everything. They should all have ability to handle the ball and all try and rebound.
Now, old saying, you can't teach height, so you probably want some taller kids on the team and not choose an all-small team. If you have the option to choose 10 tall kids and they have ball skills and passing ability, lucky for you.
So realistically, you probably end up with some small, some medium and some tall players. But I don't like the labels of guard/forward/centre etc. I've found it creates attitudes like "you're the centre, so you rebound" and "you're the guard, so you bring the ball up".
Your centre in U12 may end up being a guard in U18's and likewise your guard in U12 might end up being big, so best to expect all your players to dribble, pass, rebound and shoot.
When it comes to selection/composition, I'd make you have a combination of players with existing skills of dribbling, passing, shooting and rebounding with knowledge you will teach them all and improve them. Or even more basic, choose two teams of 5 player combination with those mixture of skills to be balanced on the court.