I just saw my daughter in law with my 3 year old granddaughter on a stand up scooter?
How old does the granddaughter need to be to be riding on the stand up scooter (no helmet?)
Scooter rules matter a lot by state or even local rules and regulations. Also if the scooter is electric or just human powered.
Based on the state, county, city where you live they might have a community service officer at the local police department that could answers the questions for how this might be enforced locally. I do not think there will be any place in the US that says openly it is ok for a child under the age of 18 to not be protected by a helmet at all times.