In this book, Lowry never actually tells us what the special training for a nurturer is. So we will need to try to think for ourselves about what kind of special training a nurturer is likely to need.
It seems likely that a nurturer would need special training in understanding the physical and psychological needs of young children. For example, when we are first introduced to Gabriel, we hear that he has not been sleeping and eating properly. Nurturers would need the kind of training that would allow them to diagnose and treat the problem. This means that they need to have some amount of medical knowledge like a pediatrician would have.
With that in mind, you should try to imagine what the training would be like. It would likely include quite a bit of book learning on the subject of the physical and psychological health of children. It would probably also involve a great deal of hands-on experience so that the nurturers could apply their book learning.