The green wives seem to be a higher class in Gilead society so their children won't be taken off them for child brides. Nick's wife said she was from a farm so a lower class in that society. However Nick is a driver and could be potentially promoted but is still lower down than the Waterfords but higher than the farmers, delivery van drivers and normal people.
There's a whole section of society that we've not really seen- everyone that was dressed in grey, the normal people. The bread delivery man and his family and everyone in that town, on the train etc.
I don't think Hannah will be a child bride- she is too important as she is a child of a high ranking family and will be sold off to another high ranking family. Not as a prize for lower ranking individuals.
Its all very medieval and everyone is in their place.