A very simple project is a big 4 patch. Use elmers washable glue sticks instead of pins; most kids are afraid of picking themselves with the pins. The glue washes out, so that is not a problem.
Wide strips are also good. My grandson liked the "new fabric" he made when he put strips of two fabrics together in a quilt top.
Probably the most important thing to teach beginners is respect for the machine - keeping their fingers away from the needle, turning the machine on and off when they thread the machine to prevent accidental pressing of the pedal while their fingers are in the danger zone, etc.
I don't let my 'grands' cut the fabric with the rotary cutter. They can watch, but I've cut myself a few times even though I was being careful. It's just too dangerous.
Good luck. And be sure you have insurance to cover you if there are any accidents!