Do separate conditioning drills for arms like rotator cuff exercises to protect your arm from overused injuries. Increase in core and leg strength will help on pitching strength.
I missed the point about her being 8U...My DD is 14 and had hit a plateau, so we introduced the weighted ball. She works with a ball that is 2 oz heavier than regulation, a regualtion ball, and a ball that is 2 oz lighter than regulation. 10 reps each going from heavy (75% strength) - regulation (100% strength) - to light (100% strength) AFTER thoroughly warming up with the regulation. It has helped out with her velocity.
I certainly would not condone this action for an 8U player...like I said, I missed that slight crucial confused piece of info.