Thank you for your question. When the team moves from 10u to 12u, big changes happen with the game. The biggest is the ball size increases from 11" to 12". In addition, the pitcher moves back from 35 ft to 40 ft (and possibly soon, 43 ft, which is what is used in college).
In addition, stealing bases, when a base runner can leave the base to steal the base, and dropped third strike become allowable in many leagues. This does vary from league to league, but generally speaking, this is the case.
The reason for the increase ball size is due to the hand size of the pitcher. Not many girls, especially as they are just learning to pitch, can grip a 12" ball, much less have good control over where it goes. As a result, softball does not go to the 12" ball until 12u.