I taught my daughter to sew at 5 and her first quilt was a rail fence. I found out strips like that are so forgiving if her 1/4 got off and it was quick. I also used very fine pins to pin her blocks together and let her sew over them, less concentrating on lining up the blocks together and more on feeding them through.
Now she is a pro at 8 and can turn on, set up the machine and iron away on her own. I handle all the cutting since she is still so young.