i would just explain to your daughter, she is not to talk to you or you bf or any authority figure for that matter with disrespect PERIOD. i am not in the place to go along with these comments about you taking your daughter around men you are dating, i did that to, but my daughter was VERY young and probably doesn't remember most was still unfair to her. when she makes these comments, ask her gently, who told you that, or where are you hearing this from so you can get to the root of the problem without making her feel guilty. if his family is talking negatively about you in front of her you can hold that against him in court, he nor you (not saying you do) should allow ANYONE to talk negatively in front of her about the other parent......most decrees default to have this listed
just food for thought, be careful about who you do and how soon you do allow any men to meet your daughter, children cannot pick as easily as adults can who to attach themselves to and who not to, if she gets too attached to a man you may not work out with it will tear her up until she's old enough to understand AND if she remember's them she'll ask you later and possibly bring back memories you'd rather not visit.
also if it's listed in the decree that no one else is to talk negatively about the other parent and they still allow it, you can hold the opposing party in contempt of court....as long as you can prove it
good luck