I don't think taking an 8 yo out in the yard, or in the garage for 20 minutes a day showing her how to throw a ball, underhanded, or overhand for that matter is something that is going to lead to overuse injuries. We're not talking about a regimine of 100 full blown 100% pitches here. I think you have the wrong image in your head of a drill sergeant screaming at a young child to work harder, throw harder.This board, which I have read a lot, has had many discussions about overuse, along with national sites and experts related to youth softball and coaches in my area.
If you want to be Kenzie Fowler (and others we have never heard of because they did not make it past 14u), fine, go ahead and get started on a path of overuse.
It could lead to burn out and the possibility of ruining your relationship with your dd.
Pitching coaches have a professional responsibility not to demand this stuff of 8 year olds. Nothing is up to the 8 year old. Please be adults.