At the 10U travel level, unless you are throwing 52-56 you will need to locate the ball, location location location and have an effective change up, even if you dont throw it for a strike all the time, just let the other team know you have one, and the need to look out for it and respect it, most young girls dont know how to handle outside pitches at this age. My DD threw hard, she threw 55-56, very effective but against top teams we had to mix in other pitches, a good change up is a young pitchers best friend. Moving up to 12U speed is really not a factor for a first year player especially, you have to spin the now, that fastball will get banged.
How many kids throw it 52-56 at 10u? Jennie Finch's top speed at age 10 was 53 and that was probably a one pitch max and not a cruising speed number. In my neck of the woods PA/NJ/NY/MD/DE/VA, of the kids I've seen at this age, the best, might hit 50-51 if they are "older" 10s (meaning Jan/Feb of their birth year). Maybe there is one kid in the country who could hit 56 (again, probably just a one pitch max, not in a game) in 10u, but I'll file that in the "I'll believe it when I see it category".