Real quick my DD is petite, 70lbs roughly at 4' 6". 10u she swung 2 bats at 29/16.5oz and a year later 30/17.5oz so she's been in the -12.5 area because of her lack of size and strength/mass. She was a slapper during that time so we needed the extra length. Both of these bats were Miken Halo's and averaged +1oz from the sticker.
This is her last year of 10u and we just bought her a 30/20 Louiseville LXT to finish it off. She's not weak by any means, just small. Our LXT came weighing in + .5oz from sticker so much happier with that.
I also agree that 10u is usually 30 or 31 on length and weight really depends on how strong your girl is. General rule I hear thrown around is -10 by last year of 10u going into 12u no matter the size of the girl. You need more mass against the bigger ball. At least once a week I let her swing a wood BB bat 30/22.5 but it really messes with her balance so we use it only on hesitation style hitting drills where she has some room to control the bat. We do this for about 20-24 balls before going back to her normal bat, again this is to train control and not speed or brute strength. I haven't seen any negative swing effects but we're not increasing this work either.
For girls under 12 I recommend Demarini, there light and have plenty of "Pop". I Coach a 10u Travel team and every girl swings some kind of CF6,CF7 or CF8 either -10/-11 between 29-31 inch. Love the Xeno/LXT for older girls, just in general a little Heavy(not all) for young ones.