Wendigo and I talked about our kids at one point. Runic Wolf and I have a 10 1/2 year old and Wendigo has a 15 1/2 year old. When his son started dating last year, Wendigo mentioned that right now is not the time to let his son know that he's poly. He thinks it' more appropriate to wait until his son is college aged and only if his son brings it up. He certainly doesn't plan on telling him that he's bi either as he feels that now is not the time where his son needs to know what his parents get up to in the bedroom; not when he's just starting to develop those types of feelings of his own.
Still, now that your kids do know. I wouldn't advise leaving your girlfriend. I wouldn't force your kids to spend time with her, but I agree that important people in your life shouldn't get thrown away just because someone is upset, even your kids. Life is hard, it isn't fair and while we should love and nurture our kids, we need to teach them that lesson too. They may want normal parents, but they don't have them and honestly no one does.