If the toddler has two functioning parents, that know their babysitters have alcohol problems, and are making a decision to leave their toddler with them from time to time, I'm not sure there is much you can do. They must feel like they know the people well enough to keep their child safe at the times they leave him with them..
If you have more information about the child not being safe, then I would share that in a very non-confrontational way and do so one time.
If this is a case where one parent is alcoholic and one parent is co-dependent and in a bit of denial it is a different thing. Also one in which I have more experience since I was that co-dependent in denial spouse. My husband has drank for many years and, even now in hindsight, I do not think my children were in any danger when being left with him until the last 6 months. Things got worse and I should have put them in daycare 6 months sooner then I did. If I would have had someone close to me come to me and kindly, gently, non-accusingly discuss this with me, I might have actually been relieved. My world was so filled with self doudt and confusion that one other person collaborating my 'gut' would have been a push in the right direction. It is a fine line because for go about it to bossy or strong and walls of anger and denial will be thrown at you, fueled by shame, guilt, and fear of what is going on, what was allowed to happen, and what might have to be done about it.