Someone has already suggested a simply draw-string bag with no lining. This is something all kids love.
Whatever you makes, do not be too critical! My granddaughter made her first quilt when she was 8 or 9. It was 4" square blocks. She had more fun laying them out on the floor to get the exact pattern she wanted. I taught her to sew the rows then put the rows together. I stressed making the corners meet until she looked up at me and said: "Gramma this is MY quilt, I like it this way". I quickly learned to keep my mouth shut!!
Sara is now 21 years old and still loves to sew. She often goes into my sewing room and comes out with an open rotary cutter in her hand -- chews me out for leaving it open on the table as I always told her if she didn't close it as soon as she set it down, I wouldn't let her sew anymore that day. She learned -- guess I haven't.