I had my first family matters court matters delt with on tuesday, the ex turned up with his new gf in tow, and his parents, i had to represent myself. His solicitor came up to me and started to talk about the ex's new partner. so i said look im really sorry but she had nothign at all to do with my children or my family so she needs to keep out of it all. Off the solicitor scuffled to give him and her feed back! deary me the arguments started lol the parents left. Initally my son had asked to see his dad tue, thu and every other sat, the ex couldnt stretch to that! apparently far to busy which i made these feelings known to the solicitor, in the end we came to the fact that he should have my son and my daughter when she wants to go tue 6 - 8. with the view that his gf does not see my children at all.
This is a common problem. The ex brings in a new GF straight away and expects everything to be hunky dory between her, the kids and you. The kids have feelings, and views. The right thing to do is for him to step back, remember that contact is a right the children have, not him, for their benefit not his, and concentrate on their needs. She is already meeting his!
If this thing with new GF is going to endure, it would be sensible to introduce her gradually later - she is a facet of his life, it's artificial to insulate the kids from her altogether. But not straight in at the deep end early on.
These things work out fine with patience. If he is too pushy he will ruin everything. His sols will know that if they have an ounce of competence in family law. Good luck!