Boyfriend with kids...

My boyfriend has 2 little girls age 6 and 4. My boyfriend's ex has custody and my boyfriend gets his girls every Saturday and Sunday. I have no problem with children. However, I do have a problem with his children. They are rude and disrespectful. They have horrible manners and don't listen nor do they understand the word "no" and when it's said they cry. I am not the first girlfriend since my boyfriend and his ex split. My boyfriend also tends to ignore me when his girls are there. If I bring these concerns of mine to my boyfriend he excuses the children's bad behavior because of their age and the fact that I don't have children so I "wouldn't understand". I have always been around children and don't feel his girls behavior is acceptable. It's to the point I don't want to spend weekends with my boyfriend when he has his girls. What do I do?!