I use the extra soft, polyester stuffing like the one used in quality stuffed toys. There are other grades of polyester stuffing, too. 100% wool and cotton are available if you would like to keep your materials all natural, for example. Check out what is available at the fabric store near you.
To the grown-up
Read about how to prepare projects for beginners.
As the grown-up, when you prepare a sewing project you get to choose where the opening for stuffing will be.
Leave about a 2-inch space blank (without dots). You can make dots at the opening after your child finishes stuffing the shape.
Leave the opening on a straight part of the softie when possible.
Leave the opening near the bottom of the shape when possible.
Tip: One way to alert your child that it's time to stop sewing, make a lock stitch and stuff the project is to use straight pins with red heads to act as tiny stop signs at the opening.
Tips for your child:1. Pull little pieces of stuffing slightly apart to fluff it up.
2. Stuff the parts furthest away from the opening first.
3. Stuff the smallest parts before the big parts.
4. Use a finger, or an unsharpened pencil to gently push the stuffing in place.
5. Use several small pieces to fill a spot rather than one big piece.
6. Use enough stuffing to make the softie as fluffy as you like, but not so much that it strains the felt or stitches.
7. Close the 2-inch opening with a freshly threaded, knotted needle using whip stitches. Lock stitch to finish.