NAPTIME!!!
Your oldest is 6, so I would try to get as much schoolwork done as possible during naptime.
My kids are 8, 6, 3, and 1 1/2. . .and I'm trying to take my own advice on this. My 1 yr old dd is constantly climbing up on the table, crying, and playing with the computer etc. . .
My big kids can work atleast twice as efficiently when it is quiet and they have my full attention.
Oh how it would be nice to be done before lunch. . .someday. ;)