When my Daughter was about that age I bought her a Suzuki JR50 motorcycle which she rode exactly once. Too much power and she bailed off never to ride again. So here's a suggestion: install a 1/4 inch headphone jack on the rear of this vehicle, break the power connection and wire it through the jack. the plug that mates gets a string attached to Dad or Grandpa. short the inner to the outer conductor of the plug. when it's plugged in he has power, if it gets away from him, the plug pulls out and it stops. You could get creative and use a shorting type jack so instead of stopping it switches to low power. I doubt
Dale Earnhart JR's. first ride was souped up very much.

RULE 1, SAFETY FIRST (Especially with kids)
RULE 2. Go back to rule 1