I think what you are doing is fine. Another thing I have done with those that can't lay quietly and not disturb others is tell them if they bother the others that there is no "special snack". Special snack in my home is afternoon snack that is a treat - so cookies, granola bar, pudding etc. Those who have lost their "special snack" privilege get something no so fun - vegies, rice cake etc.

Does the child have a portable DVD player from home? Could they watch a movie with headphones on in your playroom?