I may be wrong but I think that you just have to take what you can get from him. Have no expecations and be happy when he does make an effort to see the kids.
I think the fine line is keeping the availability open and maintaining boundaries so that you are also standing up for your own needs. ie. You don't want to put yourself out just because he wants to see the kids at 3 am., so you keep the healthy boundaries to available times that may work for both of you. Sometimes you have to be the bigger person for the sake of the children, but don't expect the MLCer to be.
This is normal for MLC. Some MLCs choose to have nothing to do with children and some almost fixate on them, while leaving the spouse behind.
The MLCer may just be seeing the children out of guilt, thus bringing them back after a couple of hours. They want to be there, but don't want to be. It is a very confusing time for them.
It is very frustrating and I feel for you and your children. I am fortunate right now that my MLCer wants to have a relationship with our child and just wants me out of her life. That also may change tomorrow.
Welcome to the rollercoaster. Can I punch your ticket?
M:35 W:33 M: 5 yrs. Daughter: 2 yr .7/11/10 D Final: 8/7/12