DS is 4 and has a balance bike but no scooter any more. DD is 2 and has his old scooter but cannot go very fast because we hold onto her scooter strap, though she can of course topple over on the pavement.
We make him wear a helmet for cycling but we should probably make her wear one for scooting. She wears one for riding in the baby seat on my bike though.
However, and I absolutely take everyone's point about fitting them correctly and I do know how to fit them, but DS has sensory issues and it is a toss up whether he will accept the helmet as tight as it really needs to be on any given day, or whether instead he will a) have a tantrum and hit and kick me/DH or b) refuse to put it on and also refuse to walk pushing his bike, instead just stopping in the road miles from school/home.
If we are out and about (or on the way to school) and the alternatives are: stand in the street for hours, or put it on loose and let him ride at only just over walking pace on the pavement, I usually choose actually going somewhere. If he won't put it on at all, we just stand on the pavement unless he'll push the bike.
We also try to get him to get used to wearing it by going a short distance to the park (again, not much over walking distance, on the pavement) because we previously got into an impasse where he would not put it on at all without violence, and said he was never going to ride his bike if he had to wear a helmet. Again, we wanted him to actually get used to the idea of wearing it so that we can work up to wearing it properly fitted.
We are not going to let him ride on roads without a properly fitted helmet. But do bear in mind that children who are wearing them but badly fitted may find them unbearable when they are tight. For us, getting used to wearing it is in the long term worth it. Not wearing it at all makes him think he never has to wear it.