I sympathise - and it IS worth really planning - have just been through it. If you think the two of you can put on a good performance together (albeit with integrity) then that's got to be the best way. If you think it is impossible it's better, though not ideal, to tell them on your own.
How old are your kids? Ours are nine up, so this may not apply to you...
I found it helped to make a list of what we know (dad's moving out, to here, you will see him xyz, your room is / is not ready, we will still be a family, we will still love you, this is not your fault, it is because we are finding it impossible to get on, we are hurting each other, we need to have some space to try to work things out etc etc) and what we dont know (how long is this for? does this mean divorce? will we have to move?). I tried to think of all the questions they were likely to ask and to plan honest, simple, un-wordy answers.
It took a long time to get their dad to play the game (esp with questions re drink, like you) but I appealed to the fact that this was something we had to 'pull together' on, that for the sake of the kids we must seem calm, stable, organised, able to talk to each other, able to put them first, able to be honest and straightforward about our feelings, too.
I planned to leave lots of spaces for them to react and ask questions, kept in mind how important it was to reassure them over and over again, to listen etc. When it came to it, one walked out and one put his fingers in his ears. The 'conversation' was over in 2 minutes. But I'd also persuaded their dad to stay here for the whole weekend so that they could go on asking questions in their own time.
It was horrible, incredibly draining, terribly upsetting. But we got through it - (are still getting through it, and their reactions to it, day by day) and you will, too.
It's really important to keep bigging up their dad - and hope that he will support you similarly. He may not. You have to live with that and keep being positive about him. For their sakes.
it's great that you're putting your kids first and thinking about how to talk to them. It can be really useful to go through it with a therapist, too. I wish you all the best with it.