I agree with answer number one but would like to add some further explanation.
The children in the 1600s were considered smaller versions of adults, yes, and the idea of "childhood" being significantly different from adulthood had not yet come into existence. They had jobs to do, appropriate to their skill levels, just as adults. The roles of women were to take care of the house and children which the girls took part in while the boys learned a trade and assisted the men.
Another important point to consider is that these paintings from the 17th Century were made as a record of existence for the family. Not all families could afford a portrait, but those who could were proud to have their lives documented.
Children were presented as smaller versions of their parents and of adults. Well-mannered and obiedient with a kind of perpetual kindness.