I sew with my niece who is 9. We have made doll washcloths (4" squares) which was her request, something I never would have thought of. And a placemat, again her request and something I never would have thought of. My now grown daughter always loved getting a home made pillowcase for Christmas every year, it was always something she looked forward to when she was younger.
I agree with sewitalltogether, ask her and see what she wants to sew, as I learned from my niece, it might be something you never would have thought of.