They *are* dangerous, especially for children. Last year, the US Consumer Product Safety Commission estimated that there were 27,600 scooter-related injuries in America, and around 85% of these injuries were to children under 15.
The most serious accidents result in broken bones, skin abrasions and lacerations. Accidents were likely to happen soon after the children were given the scooters, and younger children had the most injuries. The reasons for the scooter accidents were: going too fast, hitting an object on the pavement, and inability to brake.
It didn't matter whether they were with a parent or not, and while helmets and other protective gear protected the area it covered, the injuries covered the whole body.