Problem is, if he were to go to court to get pr, he would undoubtedly get it, so it isn't something you could withhold for any length of time anyway.
Perhaps the way to deal with it is to point out that linking the two issues is obviously his way of making the children a commodity to sell or buy, but that, if that's the way he wants it, then fine. Probably won't help you, but it might make him feel a bit shabby.