It is advisable to ensure that children are wearing comfortable and breathable clothing when they are traveling in a child restraint system and that they are not only wearing their swimsuit. It is just as bad to be overdressed as it is to be unclothed, as the child can be chafed by the child seat, especially if the child restraint system has to act.
Moreover, in an accident or when braking sharply, the only contact between the seat belt or the harness will be the child’s own skin, which could cause burns. The same thing happens to adults when they have a seat belt on and are not wearing a shirt of any kind.