I don't think there is such a thing. Why not build some kind of box to set the foot control in so that they can only press down on it soo far.

Personally I would give a demonstration on how to use the foot pedal and explain why you go slow. A child who is old enough to learn to sew can surely be reasoned with enough to undr

My machine allegedly has a dial (or something) somewhere to control the speed, and it was a reasonably cheap machine. You might want to check your manual.

My Kenmore machines have a high and low speed, but I think there is a way to put something there to keep the speed at a slow pace. Good luck. Nice to teach your granddaughters to sew.

Sue