At young ages you need to focus on mechanics and technique. Weighted balls are likely to cause poor mechanics which will lead to injury. Let them throw and hit regular balls and they will be encouraged by the distances they can throw and hit. Build on that.
I would not use for younger girls either, maybe 10u and up, and only if girls are physically ready for them
related questions, thinking of trying these with DD (12u) to improve her power at plate, do they work for that? what size/weight is reccommended for hitting with these, dizzying array of options on amazon? Should I have her use something other than her composite game bat (we have a couple of older dead ones lying around)?