Once rid if you wash your childrens hair in shampoo and conditioner that is for coloured hair, this keeps them at bay too. We suffered so much with our eldest daughter who just seemed to be plagued with them no matter what we did and someone suggested the coloured shampoo and conditioner and she has been nit free every since. This was after a lot of combing every day, you really have to spend the time getting them out.
Of course if you don't want to do that then at the very least always use a conditioner after shampooing or even a leave in conditioner.
Their horrible things but perserverance and you can beat them :o)