'They' move on to find a 'drinking partner', to find someone that won't criticize their drinking as they drink also. "They" move on to find another enabler, so that they can continue the life of numbness that they want.
But it never works for long.
I say this from my own personal alcoholic experience.
Therefore, please do not allow your children to go over their for visits. For now it may be better to 'suffer' his presence periodically when he visits the children at your house.
J M H O
Love and hugs,