I have "heard" that young children work in sewing factories in some countries - whether that is true or not - I really don't know.
I think we sometimes under-estimate what our kids are capable of doing.
Editorial comment: I also think we are keeping kids - and some adults - dependent waaaaay too long.
You know your kids better than any of us do. Some are very competent/capable at a very young age - others not so much.
I like the idea of letting them "practice" either on paper without the thread or "just making the machine go" without a needle or thread in it.
Then give them "real" pieces like Mommy uses and let them do whatever they want with them.