Aggressive base running. Teach them to RUN towards first on 3rd strike (dropped or not) and 4th ball. I get them to run on all 3rd strikes so they get in the habit of running that way. I have my first base coach call them off if it is not D3K, but it beats yelling "RUN! RUN! RUN!" which usually alerts the catcher to throw them out. Have them steal on first pitch to next batter. If you have a runner on 3rd and they get walked, have them go on to second fast.
I also teach sliding a lot in 8U (even though they don't get to use it much, they still learn it), but do it some more in 10U. When the girls get to 10U and beyond some of them begin to get apprehensive about sliding. Teach it young and they won't be so afraid.
The pace picks up a lot. My oldest played on an 8U rec. team this season and late in the rec season started playing on a 10u TB team. The 10u was a big change. But she did really well. It drastically improved her play in 8u. She had always been a great batter but after two tournaments in 10u she she hit 6 home runs at her next 10 bats in 8u coach pitch. All fundamentals must improve at that age to be really good. I was blown away at how good some of the 5'10" 11 year old girls that pitch 50 mph are.
lots of changes, not sure if you have pitching yet, but catcher is just as important. Start teaching your catcher how to defend bad pitches (probably will see a bunch), work throw downs very extensively as well as pass balls. Your catcher(s) can be the best defense on your team.
Teach the bunt and defense of the bunt, it can win or lose lots of games.
Work on base running, it can be the biggest difference in wins and losses.
Fly balls to the outfield, if not regularly caught should be worked on extensively.
Cut off and where to throw will be major for the next 2 years.
Start teaching bunting and get them good at it, in game situations 1-9.
In my experience the pitchers and catchers make the biggest difference, moving runners and teaching strike zone is the next.
Situational defense is important because depending on your league everything can change, USFA has hesitation as well as most do, this was a major rule change for us.
Teach your defense when to throw home, not just to first for the out.
You can't work first and third base fence catches enough.