For someone that does not have a road-map as to how to go I think you are doing a marvelous job. It does come at great cost and anguish to you I know. I'm afraid you do have to expect to second-guess and even modify your actions. You do not have direct experience with ice users to draw on.
I know you are allowing supervised access to your children for now and can well understand the reasons for doing so. It is true that cutting him off completely does stop him from seeing normal life and what he is missing. When unaffected by the drug he may well be hurt and full of regret at separation. Limited supervised access to the kids may well be beneficial to him. It does however come at a huge risk, and your kids are the ones it will affect - or you. Even if he never becomes violent but merely exhibits bizarre behavior that can leave a lasting impression.
The husband you knew, how he thought and behaved is no longer always there. Ice has unpredictable and far reaching effects, not only on his weight, but can also be on his thinking, beliefs and fears. The resultant thinking does not always fade away as the drug wears off.
Frankly if it was me I'd seek advice before allowing such visits, particularly as you would not be not strong enough to restrain him if he became violent by yourself. A
support group or a professional with experience of crystal methamphetamine addition (not just friends or family) can probably fill you in better about what you are dealing with.
As you remarked yourself you are choosing between your husband and your children. Although you will have thought of this yourself I'll mention it anyway. This is all a result of your husbands actions, not your children's. They are entitled to the protection of the both of you. If your husband has feelings for them I'm sure he would agree in his calmer moments that at this stage he should keep - or be kept - away.
Going cold turkey, as he did in he past, is only partly effective, as without ongoing support it is easy to relapse, as he has done. I would realy hope he is willing to get into a rehab program, and if fortunate enough to be admitted and then cease using the drug continue on with long-term support. At that stage a rethink of what you are doing would be appropriate.
I'm sorry to have to paint such a dismal picture, it realy is up to your husband to remedy things.