For a 7 year old you'd think you could leave them to play for a few minutes on their own. I have a 7 year old and I can trust her to play well in the playroom while I make lunch. I check on her every couple minutes, and I can hear everything. Most of the time she wants to hang out with me though, and talk, and talk, and talk
Obviously I couldn't leave the two year old to play in there alone because she likes to dump, throw, and go. It depends on the age of the kids and if they've shown they can be trusted. But I'd certainly think that by 7 they could be trusted to follow the rules of the playroom and play independently in there for a few minutes. Thriftylady, do you have your toys organized into "centers" (areas) for the same type of toys? Like a Block center, housekeeping center, puzzles and manipulatives center, etc? If you do, it may help to take pictures of the toys and equipment (or clip art of that type of toys) and put a picture under each toy to show where it goes. For example, for the basket with mega blocks, you would hang a picture of mega blocks to that basket. For the shelf that holds the puzzles, you would put a picture of a puzzle on the space of that shelf where the puzzles go.
When I first did this for my class at a preschool I used to work at, I figured at clean up they would still just throw everything into piles or wherever they saw room. But I walked them through the centers pointing out the pictures and explained how it worked. I had to stay right on them for a couple days at clean up, but within the same week they were already putting the toys in the correct buckets an baskets and shelves. Pretty soon I could walk away to get lunch ready and leave them to clean up on their own. I had ages 2 1/2-5 in my class.