Been on both sides of a girl who only could pitch 35-37, but had good control. With a good defense behind her its definitely worth pitching her once through the line-up as a relief pitcher or starter. Good hitters at 10u (and sometimes 12u) have trouble adjusting to slow pitching when they are used to mid-40's or faster.
Heck, my DD's 12U TB team had that issue in a tournament 3 weeks ago. Low 40's from 40' resulted in our girls hitting weak grounders to the middle IF all game long, and we got eliminated by a team we have never lost to. It was a sad day.