My girls have always been fascinated with sewing. They began begging to learn at an early age. Not a surprise, of course! Now they love it so much I want my machine back! Teaching litle kids isn't as daunting as it seems!
First teach safety. Yada yada.
Next teach how to raise and lower the foot, backstitch, press the foot pedal, etc.
Finally, give him or her a scrap and let them scribble sew. Easy!
I don't see the point in restraining their creativity while they are young. They don't need to think sewing has to be a certain way with straight lines and perfect corners. In fact, they'll probably surprise you by what they come up with!
This is a typical scene from my sewing machine. My 3 oldest and the neighbor girl. All taking turns digging through scraps and sewing masterpieces.