Bat them all unless the rules prohibit it.
This is 12u fall ball, play them all. They all need to play to get better and the only way your team gets better is if your players get better. At 12u we only batted 9 when the rules required it, which was one state championship tournament. We did the same at 14u, and are doing the same at 16u. We like winning and we do quite a bit of it. We won five tournaments last year and batted the roster in three of those. The only two we didn't bat the lineup were two state championship tournaments and everyone played in every game of both of those. Winning is great, but we know our job is to develop softball players.
Thanks to the good information here and open-minded coaches wanting to have a fun, engaged group of 10U girls, we've made big improvements. Thank you all.
For other new TB coaches:
We went into our first tournament and started our best lineup for pool games and bracket games. We finished second, girls were ecstatic, we all had a great time. Then the phones rang, with a few parents upset their DDs didn't get infield slots and batted too low in the order. We told them (before they joined the team and again after the tournament) that this isn't rec ball and we're playing girls at their best positions and positioning the team to win.
But we recognized we could do better.
Last weekend we used what some here called "The Wheel" for the batting order. Game 1 ends with batter #6 making the last out, so Game 2 starts with batter #7 leading off. We did this in pool games and we even kept this going through the bracket games. And we told the parents how this gets girls essentially equal number of at bats and lets different girls lead-off, bat fourth, etc.
Also, in pool games we had our secondary position players play the position at least half the innings. Yes, our errors went up (both physical and mental, especially). But they were learning opportunities and really let us coaches determine our talent. And the girls had fun and I imagine the parents are appeased some.
And we won the tournament.
Feeling pretty awesome from the coaching side. Again, this message board (and having fellow open-minded coaches on the team) helped a ton.