I would suggest you get some 'peppy' music and at least every 2 hours, maybe even more often have wiggle breaks where they exercise/wiggle their necks, shoulders, arms, backs, backsides, and legs.

I think you are right to have things pre-cut. Yes, it would be great to teach basics, but it would require more than one day. This will give them a taste of quilting, and what they really want is good food and a finished product to take home. How about a quillow, with big blocks, and making a sandwich to finish? I also like the idea of a rag quilt. Find a local guild that would be willing to donate batting scraps. They may also be willing to help pre cut or even come and help for the day. Just some thoughts.

I have taught children summer sewing classes...
1. Yes precut.
2. Have one adult for every 3 children---thread breaks, need to rethread machine, margins too wide, need to seam rip etc.
3. 9 to 5 is too long. Could you make it 2 session?

I is so much fun and if you don't try to get too complicated.

I did a day with Ferret and finished with a quilt . I was slow and did not bind mine. The easy quilt was called a baby quilt it was about 24 inch strips in the centre surrounded by a border and more borders . The picture is mine which I gave to the dogs they love it and I hate my colour choices.
It is quilt as you go therefore you cut top back and wadding same sizes. A walking foot was used.
Place first piece on table sandwiched in correct way. Put backing to the backing right sides together ready to sew seam . I would pin at right angle to seam. ..now right sides of together of piece first and second. Move pins to include this piece. Put batting on top of second and move pins.
Sew a. Seam. .. You may wish to trim batting but press . One section done. You do get quicker.
Next is to quilt that section .
Ready for section three . If they are fairly large. The middle medallion is 15 inches.

I also made a doll quilt size sorry cant find it.but it did work if you are experienced in quilting.