Relationship concern: Resolution without conflict

My boyfriend and I broke up from 8/19-10/19, so two months, and when we got back together he didn't tell any of his friends or family that he did so. He continues to hide this fact. He says that it would cause him drama and stress. He says he's worried someone would tell his ex-wife and she would make his life harder or not allow him to see his children. Apparently his ex-wife thinks I'm awful because of things that his kids have told her. Mind you his son has chose to hate my child since they met when my daughter was freshly 3 and his son was 7, his daughter was 5 but she gets along with mine alright. Mine is now almost six, so it's been almost 3 years. Admittedly, I've had mama bear moments where I was defensive of my child with his son. But my boyfriend also told me that his kids told him this Christmas that it wasn't me they didnt like, it was my daughter. Either way, I feel upset and do not know how to talk to my boyfriend about it and not end up fighting.