Really work on their movie concentration time. Buy a dvd loaded with older "safe" but engrossing cartoons that you only let them watch as a special treat. We also have this wonderful video full of classic stories that runs for 6 hours (they don't watch it for 6). If you find a movies or 3 they really love, don't let them watch it except at times you need to leave them only indirectly supervised/. Have a bunch of special activities they can perform unsupervised that they rarely get to do. Ditto toys.

That should allow you to create some grading spaces.

Other ideas can be to never let them go in a fast food playground thing unless you have something you really need to do (readin, grading). Take them for lunch and let them play an hour or so.