You can legally go up to 28 mph with E-Assist bikes and stuff. . . I personally think the 20 mph speed is more than good enough. Based on the age of the kid. . . Heck at 6 year old. . . I would recommend closer to 10-12 mph governors.

There will be very painful accidents though.

I have an E-Scooter that goes up to 32 mph. . . it is a freaking beast and I love it. but I would never let a kid ride it.