I started my youngest one at the age of 12 on a 340 Panther. The older one, 14 at the time, was on an Indy 500. The 12 year old was always screaming she wanted a faster sled..... she's a bit of a dare-devil. That sled went about 50 ish. I felt ok with that. Any 500 liquid or even a 550 fan would be a great starter sled. Things to consider, whatever you do, is comfort, warmth, comfort, warmth, did I mention warmth? At least in my dealings with a lot of women/girl riders, they always need to be warm. Make sure they have the right clothing. Get gauntlets for them, Good coat and bibs, and good boots. If they get cold, they won't want to ride any more. And make sure it has a good suspension. My oldest basically gave up sledding because the Indy 500 had the old suspension. Too bumpy for long rides.
Just some advice from someone who got his wife and two girls to ride for a few years... until other things became more important... (ehm..... like boys, cell phones, facebook, and all those important things...)
Hope it works out for you. Some of the best trips I had were taking them with me....