My husband and I are in the same boat. He left the house last April. Our children are 6 and 4. We sat them down and explained to them that mom and dad are not being very nice to each other and we need to have some time away from each other. We then said Dad is moving to a different house where they will now have another bedroom that they get to help decorate and tried to get them excited about that.
It was not the complete truth. We don't fight and their dad is a good guy. Just like what you said, he drinks too much and I felt like a single mom having to make all of the decisions and keep everyone on schedule because I did not want the input of someone who was not sober. I could not tolerate it anymore. My 6 year old is a sensitive child also. He tells me every day that he misses dad and wants to know why dad can't come back to our house. I just keep reminding him that mom and dad's job is to be the best parents we can and for mom to do that I can't have dad in the house. I know I am a better parent without him in the house. I am not angry/upset everyday. I don't check the garbage to see how many empty beer cans there are.
My husband is struggling with my decision. He doesn't understand why it is a problem for me since it is not a problem for him and he has a good job. He has no desire to change because it is not a problem for him. I hope he is or will become (after some more time passes) a better parent also.
I have found I enjoy my kids more because I am happier in general. I do have more time to myself. However, there are some days when I am overwhelmed without having a second person there. One day at a time. It also helps we are on very good terms. We both attend all kid events and he brings me money for his portion of kid costs on the schedule we agreed upon.
It was really rough for the first 2 weeks after we told the kids. They cried alot and it would say things like "my daddy doesn't live with me anymore" to random people. My older child also got upset with my husband and would not listen to him one day. When questioned about it, my son said he didn't want to listen to someone that left without him. I know it broke my husband's heart. So I sat down with my son and explained to him again that it was mom's decision and I had to do it to make me a better/happier person and it was ok to miss dad and still like him. I also told him if he wanted to be mad at anyone, it should be me.
School has made things a bit easier too because we are all busy with having to be somewhere on a schedule, homework, packing lunches, soccer, dance, etc.
Sorry this is turning out so long. One last thing I thought of, I started giving my older child the opportunity to help with more things around the house. I did not force him, but said hey I could use some help with dusting, do you want to do it and earn some money. We each have regular chores, but if the kids help do extra stuff then they get rewarded for it. He is all about helping. He even packs his lunch everyday. I inspect after he is done, but he takes care of 95% of it. I can't believe what a good job he does. I think this has really helped him because it makes him feel like he can control something.
I would make sure both of you talk to them together. We also had a family dinner on Sunday night for the first 2 months. It was ackward, but good for the kids. We have stopped now, but still see each other twice a week for kid activities.
Let me know how this goes for you. We can compare stories. :)