I radar gunned many 12-year-olds at the AA travel ball level in the East Bay of CA this summer with a pocket ball coach radar gun. This captures the speed as it comes right out of the hand.
The majority of pitchers were between 54 to 58 MPH, and many more were slightly out of that range a little higher or lower. Three times I recorded a pitcher consistently able to achieve 65 MPH and in all three instances those players were above 5' 6" tall. I also recorded several players below 50 MPH.
What I also noticed was that there was no relationship between velocity and effectiveness at this age, field size (50/70) and velocity range. Looking at the stats on our son's team:
The player with the best WHIP and ERA is the only player on the team who throws very slow fastballs at 48-49 MPH. He also had a changeup at around 44 MPH. The coaches were afraid to pitch him and basically only pitched him when they ran out of other options. He only struck out one batter all season but the vast majority of balls in play were weak popups, massively towering flies to the outfield, or grounders. So mostly those were outs.
Among the three players who pitched the greatest number of innings, the one who had the best stats had the lowest velocity (started the summer around 53 MPH but has increased to 58 MPH in August). That was the player with the best command (he could hit corners at will) and the best secondary pitch, which was a variation of a "football curve" that came in much slower and moved a bit. He was a lefty who also had the best pickoff move on the team.
We had a player who threw 65 MPH who also achieved pretty good results but he throws sidearm and sometimes teams seem to have no problem hitting his fastball. His fastball was not his most effective pitch. His knuckle curve got better through the summer and when facing good batters it was far more effective than his fastball. I think his stats were not as good as the previous player I discussed because he sometimes tried to rely on his fastball and would get hit for a few batters in a row before he switched to trying to get them out with his knuckle curve.
I've recorded stats for many years and it always amuses me to see coaches believe so strongly in velocity, even when the stats say one of the slower pitchers gets better results.
I don't have any stats on kids older than 12 so presumably velocity matters much more at the older levels, when the hitters have much more experience and can usually hit line drives off any type of slower pitcher, even ones with a lot of movement.