Ugh... really, really bad idea. Have some clinics, work with the pitchers outside regular practice, do a modified coach pitch count in games (i.e. coach comes in to pitch on 3-1 & 3-0 counts) if you have to.
If you have money for pitching machines, you have money to maybe pay someone to do some skills development.
I have been pushing our rec league to start 8U modified kid pitch - so you should be targeting going the other way - more kid pitch earlier - rather than later.
I have used my pitching machines for the past few years, after every pitching practice both of them hit about 100 balls each,three days a week. they don't work on mechanics during this time they just work on being loose and putting the bat on the ball, along with timing. doing this they really have developed fantastic bat head control and the ability to put a bat on anything. they have learned how not to swing for the fences and it translated to a 600 average over the season of exposure and A ball for both. If you are looking at working on mechanics then soft toss, Tee work better but being fluid and confident are really well developed by just feeding the machine ,Keeping you mouth shut and letting them develop the see ball hit ball mentality. From my experience batting is the most over coached discipline in softball.
For my local league, 10u plays kid pitch until 4 balls, then coach will pitch. Coach pitch is unlimited. Once the batter puts the ball in play or batter strikes out, next batter is kid pitch again. New rule change this year will be that runners are allowed to steal while kids are pitching, one base per pitch, no stealing home.
Machines might be nice to get a little more speed then what the coaches throw, but I would HIGHLY recommend that the kids pitch most of the time. Moving from 10u to 12u is hard enough.