35 ish. I had a 9 yo that came to lessons 2-3 times a week for 6 months. She was at 43. Then, she moved to 12U with the big ball, so as a 10 yo she is back to 38.
Our League's 10U pitcher evaluations included 7 girls who threw over 40 (on radar), with 2 throwing 45. Of those, 4 have the ability to reliably throw strikes at, or near, their top speed. The others are a bit more random.
We were at a tourney this weekend in Ohio and saw 5 pitchers that were consistantly at or above 45 mph.....Most of the rest were somewhere between 38-42 mph. All of these speeds were done with mutiple radar guns from multiple people and coaches.