"Sending [a] six-year-old to his room for a couple of hours on a Saturday afternoon" sounds like it could be heaven to a six-year old, not punishment. He (or she?) is in a room with all his stuff, gets time off from the siblings, and isn't being asked to do anything unpleasant.
I'd never do a time out in the kid's own bedroom, or a "go to your room" punishment, because unless the kid's room has absolutely nothing of interest to the kid in it at all -- it's not really depriving him or her of anything except maybe being outside on a nice day, and kids this age are old enough to know there will be other nice days to go outside.
I also don't like chores as punishments; how will you later get them to do those chores just because the chores need to be done to keep the household running? Scrubbing toilets has to happen, whether as a punishment or just because they are dirty and need it; making it a punishment means that later if you say, "Your chore list this school year includes scrubbing the toilet," the child sees it as punishment when he's done nothing wrong.
Your older kids are old enough to have major things taken away for serious amounts of time. What is their "currency" -- the couple of things they really, really value, the loss of which would really affect them? If it's TV time, take that away: "If you do X, you will lose all TV time today and tomorrow as well. Sorry, I know favorite show Y is on tomorrow, but that is the discipline I've set. No, we will not Tivo or record it for you to see later." Then if they do X, well, they lose the TV time, no argument, no negotiation. In our house, arguing or trying to negotiate something less than what was established can result in losing even more of the desired thing. Set the consequences in advance if possible, so they know what they will lose if they do X or what else they lose if they do Y. But you can't anticipate every infraction or make a long list of every possible consequence.
So what matters to them? TV time, computer time, games are good candidates. If they planned to be out riding bikes, then they lose bike time and have to do something else that is not a regular chore and not likely to be one. Another thing to lose for really serious things is an anticipated outing like a play date or event. You were going to take them to the mall to the Lego store or whatever? Not happening. Six and seven are old enough to "get" that they were expecting to play with Jill in two days but will now not get to do that. I hate to suggest it because it punishes Jill too, and I would reserve it only for something major, but it does -- with many kids, maybe not all -- bring home that you do not get to do nice things if you do make certain poor choices. Yes, it still affects you because you don't have the time off that sending a child to Jill's house would have given you, but it will be effective.
Make sure they clearly connect the loss of the event with their own choices. You chose to do X; I said that if you did, you would lose something you liked; you have lost Y. That is how it works from now on. It's not a matter of mom being mean; it's a matter of the choices you make.
They will beg to "earn back" the denied TV time or computer time or outing or object but I'd say, no, the loss of this is your consequence. There is no earning it back. Next time you want to have a playdate/have this thing/watch TV, you need to stop and think. But for this time, it is gone.
As for leaving a sibling without a playmate, they will survive that. Learning to cope without any playmate around is frankly good for them--they don't always need to be amused by another kid or a sibling. And it enables you to say to the sibling who is not being disciplined, "I"m really sorry brother or sister isn't able to play with you, but he's being disciplined for making a poor choice. It affects you too but he has to lose this play time so he will make a better choice next time."