I have the pigma permanent pens and they make a nice fine line but they can be a little hard to get a clear dark line. I think I would try a sample leaf with a regular permanent fine tip shapie. Sign you own name, stitch it down to a background square and wash it like you plan to do the wallhanging. If it comes out well, I'd go with those. One word of caution, lots of supervision with any permanent marking pen.
As others have mentioned, I'd use large square pieces of fabric with freezer paper ironed to the back. Draw on many leaf outlines on the fabric squares for the kids to sign in. If they goof some up, there will be others to try. You can cut out the ones that you can read and sew them on.