Probably make a bit of difference to have something separating the skull from the pavement on initial impact. More important is that the child won’t be seriously hurt in a minor accident where the bike just falls over. I know a few cases where this has happened (to adults) not wearing helmets and the results were fatal. I was in an accident myself where I lost control of the bike going over a manhole cover that was raised from the road a few inches. I had a helmet on but still had a headache for a week. I have little doubt that the helmet saved my life.