Hi A.,
First of all, I think Ellie hit this right on the mark. And I have my own two cents to add...
regarding shaving: mixed bag. My sister and I are both hirsute and had very different experiences of it in regard to our mother's concern and how she felt/what she did. My suggestion would be to find some open-ended questions to ask your stepdaughter if/when this subject of body hair comes up. "Well, what do you think you'd like to do?" or "How do you feel about it?" You might be surprised to hear what she has to say.
While it is certainly their father's place to speak with their mother, what you can do is just be there for the kids. There is a *wonderful* book that is perfect for this situation entitled "How to Talk so Kids Will Listen and How to Listen so Kids Will Talk" by Faber and Mazlish. I cannot recommend this book enough, because the principles in it can be applied to any sort of interpersonal relationship, not just adult/child. This book gives reasonable, respectful techniques that can help you be there for your stepchildren while giving them space to feel okay about their mom, and encourages them to do some of their own problem-solving. This would be my recommendation for the "call him Dad" dilemma...see what sort of solution the kids come up with, and see if they can work it out with their stepfather. Then, and only then, if this doesn't work, let the adult intervention (your boyfriend) step in.
One more thing, and it hasn't been mentioned yet: family counseling, in some form. I know from experience how difficult divorce and familial transitions are, even in the best of circumstances. All the adults involved will not always agree on parenting decisions the other household is making and there needs to be some non-traumatic way to deal with differences and conflict in the years ahead. This is in no way a judgment regarding any one party's parenting skills, but just a realistic suggestion. Kids sometimes need to have a 'neutral' third party to talk to about all of this stuff with, without worrying about hurting someone or another's feelings.
You obviously really love and care about the kids. Best wishes, and again...get that book. I guarantee it will help!
H.