I recently discovered that my husband has been having a long standing affair. This coupled with his excessive drinking and his abrasiveness is the final straw for me. My question is simply this, "What do I need to do to ensure that after the divorce my husband will be limited to supervised visits with our children?" Don't get me wrong, my husband loves our children, but when he is drinking he doesn't make sound decisions. I have come home from work to find our 1 year old daughter reeking of beer and he's readily admitted to allowing her to drink some... and this is not a one time occurrence. My babysitter has also complained that our children arrive at her house not strapped in their carseats, which we have plenty of to go in every car we own. I am concerned that if left alone with him for an entire weekend our children will not be properly supervised or cared for. I am broken-hearted about pursuing a divorce and even more so about breaking up our family... but our children's best interest is what is most important at this point.

You have valid concerns. Now you have to prove them. Without proof you may just look like an angry wife.
You could try to go for it and see what happens.

You have to document any abuses to present to the court during the custody negotiations. And just your word will not be enough.

I have pictures of my daughter holding and drinking from beer cans that he forwarded his girlfriend... I found them when I found the texts that confirmed his affair

Also, my babysitter is documenting when the children don't arrive properly restrained, clothed, etc... since it is his responsibility to get them to her every morning.

Well, keep those!
And start a journal. When he calls, what he says, if you suspect him drinking. When he sees the kids, when does he pick them up, was he on time? Did he return them on time? EVERYTHING.

Contact a lawyer. Most offer a free consult (make sure before you go) and tell them what your concerns are and see what can be done.