Is it against the law to keep the non custodial parent away from the child because custodial parent wants more money and a new girlfriend?

It's illegal to go against the court order for custody or visitation so if that is what the parents is doing they can actually lose custody if not stopping. If they want more child support they have to petition the court and ask for a modification. It might work or might not. None of the parents can demand who the other one is seeing or not seeing or who they introduce the child to when she is with them unless that person is unfit to be around children.