It seems about midway thru the MwC run, one of the producers decided that the Bundys needed a third child.
They tried to write Katey Sagal's pregnancy into the series in Season 6, but the baby's stillbirth killed that. Then, they added a child in Season 7, Seven. Unofrtunately, they fell into the "new character" syndrome, and got rid of him (eventually).
Two years later, when Ron Leavitt co-created Unhappily Ever After, they made a family with a third child, often the ignored one.
What do you think?