First things first, welcome aboard to DHC
As for the subject at hand, I am not sure what your financial situation is, but if you have the $, consider a Rem 1100 youth model. Being it is gas operated, there will be alot less kick for her, trap load is one thing, but a high brass duck load may be too much for her in a pump the next few years yet until she grows a bit. The pump could cause her the flinch-which is not good. From my view point, give the novice the best oppurtunity to succeed.
That being said, you are right on the money in considering fit and balance as #1 for her, but a close second will be kick for her. Remember you want her to enjoy hunting-not getting a bruised shoulder from it which could turn her off a bit.
BTW, my advice is after she gets the hang of going away trap shots, take her to the skeet range as many of those angle are like incoming or going away ducks
Get her away from the others, and just you and her together on that field as she is learning. That way you can spend more time at 1 station untill she figures out the lead and consistently hits the birds and then move to the next station. Just have the trapper keep track of the # of targets and all should be well will the range management.
It is good to see Fathers get both girls and boys involved in hunting and shooting sports
I'm sure a few of the other members will be adding their thoughts too.