I would prefer the golden hamsters to make good pets for a 12 year old. Dwarf hamsters are small and likely to get crashed easily, plus it depends mostly on the sex. Male hamsters are friendlier than females, which can be very cuddly.
In general hamsters are solitary creatures and do not get along with other hamsters. They are also prone to bite out of fear. Gerbils may be a better option as they are sociable and less likely to bite. They are also more likely to be active during the day rather than at night.