IMHO drop is less important than swing weight or "moment of inertia." First step to getting a correct-sized bat for a pre-high schooler is getting the length right. After that focus on swing weights.
Less physically strong kids or kids who desire more bat control will prefer bats rated closer to 60 whereas your strong boys and power hitters will benefit from bats rated closer to 80 etc.
The reason I suggest this approach is because you can have two bats with a minus ten drop, say 30/20, and the 80 rated bat will feel heavier on the swing than the 60. This is because the 60 is more balanced and the 80 is more barrel-weighted.
Read
this for a more detailed explanation. Once I learned about this stuff, I shifted some of the team bats around on our Bronco team and most of the kids improved at the plate.