I started a quilt with my 11 yr. old GD (before she had to go home over 100 miles away)... and kept the squares manageable... each to be 9" finished. We used her choice of scraps of colors (purples) in my meager stash...
Start with one patch: simple 9.5" square; next is a two-patch: 2 - 5"x9.5" sewn together; next split rail fence with 3 sections; next a 4 patch which now gets into matching corners, but simply... and then you can get more creative... when they see how one patch goes next to another (or get into those sashing strips... ) they will be hooked!!
Of course a smooth running machine(s) and plenty of thread is a must.
GD not only had to learn piecing but measuring, and some cutting and how to run a machine! She caught on very quickly but we still need to put these together and make it a true quilt... in time.
Good luck and most of all, have fun!