There is no one child policy. An Ocampan female is fertile once in her life near her third birthday. Kes was petrified that she was too young to have a baby and that it might hurt her terribly to go through with a pregnancy but pissed that this might be her only opportunity ever to have a baby.
Multiple babies and bugger all males.
Consider that Kes' father died of presumably natural causes when Kes was 1 years old, which means that her mother was four when he was 9 and dying, so he must have been 8 when he knocked up mum.
Either this is the natural pairing off, that old men do it with young girls and then the women raise their children alone over their husbands grave or... Polygamy. Each man continuously fertilises a stream of 3 three year olds during the course of his short life time.