Jellos are huge pillows filled with foam that children can jump into or climb on. They include an inner pillowcase made of sheeting fabric and a removable outside pillowcase made of a soft fabric (velour, fleece, fuzzy knit fabric). The outside case has Velcro tape for closure on one end.
The inside pillow is filled with big chunks of cut upholstery foam. You can provide it or I can get remnants from a local upholsterer.