Mid-40's is is above average for 10YO. You said she threw a 12? You mean 12inch ball or 12 spin rate per sec (RPS)? Also, RevFire is great at measuring RPS but not accurate for pitching speeds especially at 35 feet (I think they only have 2 settings, 40feet and 43feet) which makes a big difference is she is pitching from 35feet?
With that said, I would use the RevFire to measure and increase her spin rate which becomes more and more important as she moves up to 12U and 14u, and back to 40feet and 43feet.
2nd year 10u my DD was cruising at 47/48, occasionally throwing 50mph, according to pocket radar. In NorCal 10u B last summer it seemed about 1/3rd of the teams had a pitcher cruising 47/48 and 1/3rd had one throwing 44/45.
Anything over 40 MPH at 10U is very good, but I would not get too caught up in comparing pitching speeds until a player reaches 16U. Players mature at different times and the 12U flame throwing phenom could be washed up by 16U as her peers pass her by. Work on proper mechanics and hitting her spots, then add a change up!