Ceri :ghug
This is tough for you and I am sorry you are going through it.
I wonder if it might be worth drawing up an informal separation agreement with clearly defined access rights and the requirements for the capability of caring for the children? This might put measures in place when your OH is impaired by drink. Or possibly think about going to mediation to set these boundaries in place.
Of course they will not help if your OH decides to pick up a drink while he has the children but it might clarify the situation and give you some 'rules' to work with.