I have to agree with @madamecrabster. Yes, of course kids can get kicked or scratched accidentally in practice, but this isn’t routine. Swimmers who practice together regularly will know what order they should be swimming in so that no passing is needed. Kids with bad attitudes about this should be corrected by a competent coach. After 23 kid-years I can’t recall my kids saying that someone swam over them and pushed them under water. That sort of thing would not be tolerated.