when I split up with my ex years ago my kids stayed with him as it was his mothers house and I had to leave(long story) but when they got to be a bit older they asked to live with me and I went throught a solicitor who said that at 13 they were entitled to be heard in court and their opinion listened to - in other words they could say who they would prefer to be with.In the end it didnt come to this as my ex let them of their own free will but I hope this helps.