Whatever project you consider, remember that not all the kids will have an extra shirt to bring. My sis taught for years, and always had to have a few inexpensive extras (or, in keeping with the recycling theme, from thrift shop & wash first) for the dear little ones whose parents could not provide a T-shirt for their projects, even an old one no longer used was tough to come by for some of her students.
I'd nix the plastic bag filler - might melt in a dryer if the pillows get washed, could be hazardous if a younger sibling or a pet tore the pillow.
Let us know what is decided - will be fun to see what is made.