Transformed,
I think that we're in the same boat where money is concerned. I'll be dealing with STBX's inability to budget when I move out and he takes over the WAY TOO EXPENSIVE lease, and I'm really concerned that he'll be drinking his very limited budget away and not feeding my stepson properly. At the same time though, my stepson is 12, a growing boy and sometimes eats like 2 grown men, so I'm concerned that he'll throw him my way whenever there's no food around, thereby throwing MY budget off...So, I feel your pain.
In any case, I don't remember/know all the details about your ex, so I'll just ask: is his alcoholism documented? Does he have any DUIs? Has he been intoxicated in the presence of the children? I ask because if that were the case, it would be grounds to demand for supervised visitations.
With regards to changing your mind, I think you definitely can, as the thinking adult in the situation. Normally, you wouldn't give a crap if he drank away his last penny and found himself homeless (ok, well you might give a crap, but you'd do your darndest to detach and let him decide to help himself)...but since there are children involved, it is your responsibility to step in, if only for their sake.
I don't know if you're able or willing to do this, but were I in your shoes, I'd deny him visitation on the grounds that he is incapable of providing a healthy environment for your children. He HAS to feed his kids properly, and instead he's drinking the cash away and relying on Good Old You to bail him out, so he can have his cake (booze) and eat it too (kids). No go.
So the conversation could go like this:
"I know that we agreed to certain terms with regards to money and visitations with the kids, but you are ignoring the terms we set by being careless with your money thereby creating an unhealthy situation for the kids. I don't have a choice right now but to keep the kids with me until you prove that you can behave like a responsible adult".
Of course, I'd find myself a free consult with a family lawyer and verify my rights with regards to the situation first...
Ugh, what a horrible thing to deal with...*hugs*
Oh, one more thing...I work for a giant law firm, and when my relationship broke down, I quietly asked around about mediation. I was told that it would be preferable to get a mediator who is also a lawyer, so that you can benefit from their legal expertise during the mediation process...Would it be possible for you to obtain a different mediator at this point?