Bat Weights for 7th and 8th graders - What do you think?

Our league in Israel just split an age group so that now we have a group of kids in grades 7-8 and a group in 9-10. This has raised some discussion about bat length to weight ratio. Our old rule, before we split the group in half, required a -5 bat or higher. There is a lot of concern that our new "young cadet" division will have a difficult time getting around with a -5, and we need to allow them to use a -7.

As this is a safety, issue, I am curious what you guys think of a 12 or 13 year old swinging a -7 bat. Our players are not nearly as experienced as players in the US. Most of them have played for a few years, but their season generally consists of about 16-20 games, with about 20 2-hour practices throughout the year. Is there some way to test to find out the right bat weight/height ratio based on their collective skill level?