Something simple like squares, either 5 or 6 inch sqares go together fast and easy, and forgiving. Also the rail fence is fun and only straight seams, although the squares are only about 6" to keep straight when you are starting. Also, teach them to be able to thread the machine and either make the bobbins with the machine or else use a bobbin maker.
(I wasn't taught to make bobbins and almost quit because I was so troubled) Get one of those klutz gloves to protect those little fingers too. Have fun