Well, obviously we can't know what role the number of children play in anyone's unhappiness in life. In our grandparents' day, though, women had a lot of children often due to lack of birth control methods. Very different from now, when the women with large families have them by choice. Health care also is a lot better now... I'm sure a lot of our grandmothers had undiagnosed/ untreated health problems related to childbearing as well. I think I read a statistic or article somewhere talking about the profound regret that the majority of women and men experienced when they didn't have children at all. Hard to generalize... but by all accounts this family seems pretty happy.
Seems like they're very happy and is best to err on the side of being positive.