I wonder why you don't have them all do the same thing but at different levels? Why can't the oldest do art when the little ones are doing it too? Our Girl Scouts troop is K-5th graders and we often do it this way...like we will do a craft and I will have 3 versions. I don't even say it's for older or younger, just that if they are beginning level crafters, they need to prove to me they can master the more basic aspects before I will let them move to the more involved versions. Or, if I have an older girl who finishes quickly or likes to help, she can provide more one on one for a younger girl.
I don't know how your two olders get along, but my oldest LOVES to play school with the younger, who also loves it. I'm not using this formally, but it could be a great way for collaborative learning and strengthening their bond.
I agree with the suggestion of a manageable list of what you really want to accomplish. Without the distractions of waiting waiting waiting for the other kids, 8 hours of standard schoolwork can be done in half the time, if not less. And don't underestimate the importance of play, imagination, spacing out, and free reading time...
And what about the computer? There are lots of free to use websites that can make the "school stuff" seem alot more fun
