I think you probably just have to be quite firm on setting boundaries and keeping him occupied upstairs. If your older daughter goes downstairs, I think that's fine and if you're happy with that, then I'd carry on letting her do that, but you'll probably have to set firmer limits with your 2 y/o. Ours is 4 and we've only just, probably since around the time she turned 4, allowed her to go downstairs without us on some mornings. She doesn't usually stay down there for long, usually like you say, to get a drink or to get a toy or sometimes she will sit and play for a bit before she comes back up. But at 2, definitely she was too young and I wouldn't be comfortable with a younger one doing that, just because the older one did. If you don't feel you can keep your 2 y/o contained in his room if she's going downstairs, I would bring him into yours. At that age, our daughter would either climb into bed with us and cuddle for an hour or even two before we all got up or she would come into our room and play with a few toys that were in there before we fully got up. That worked and we always knew were she was and what she was up to and no concerns she was sneaking off someone she shouldn't have been. Because of the age difference between them, you'll always been having different rules and expectations, so I think probably good to start laying the groundwork and being firm about it now. If it's too disruptive, maybe one of you gets up while the other sleeps and you switch off the other weekend day.