Custody of children help?

My bf has 2 children from a previous relationship but has been having problems getting visits with the children because the mom has an irrelevant issue with me I guess. So she holds the kids above his head until all attention is directed toward her. He basically has to lie to her about me just for him to see them which is just very immature. We're now talking about taking this matter to court so at least no matter what he has rights to the children whether she's on pms mode or not. Where would we start? Funds are short but help is needed.