Yes, you need to have everything in writing before you make any changes in the kids living arrangements. I am not an attorney but you need to seek legal advice to make sure all your basis are covered.  You do not want your soon to be ex to charge you with kidnapping.  Try and work it out with him calmly if he will allow it.  Good Luck to you.  Remember "honesty is the best policy".

Sorry this may not be the answer for your question but if he loves his kids why do you whant them away from there father?Would you whant to be away from your parents when you were young?I don't know if you still love him but if you do and he is putting his heart and sould in to it for the first time of his live.I belive its worth it.I love my wife and I din't value her the right way and i allways found fault in everthing and that was a jerk of me.My wife has a protection something by the law and I can't see my kids.I love them to death.I'm sorry but I'm going to hard time.I'm only 22 and i acted like a child.I'm sorry for not answereing your question right.But does the kids love there daddy and are you living with your new husband or boyfriend?If you only know him for about 2 months or soo be careful.