Side airbags are not dangerous for children if they are properly seated when traveling.
On the other hand, since side airbags are designed to protect an adult, in certain cases the protection that side airbags offer to children can be very reduced. For example, in the case of curtain airbags, these are usually too high for children, so it is still possible for their head to hit the door during a side impact. On the other hand, the other main type of side airbags, those designed to protect the thorax of adult occupants in side impacts, can be too harsh on the fragile thorax of children.