There was a manga about that, only the partner-swapped newlyweds each only had one child... who then fell in love with each other and married. Also, totally against reality, all four just liked each other so darn much that they all decided to live together as one big happy family.
Wasn't that an episode of 'Lewis'? Only there I think there were 2 kids per couple.
No idea, but it is a manga/anime called 'Marmalade Boy'.
A similar family showed up on a Family Court episode twenty years ago - the judge was reprising cases from his tenure in family court.........a young couple showed up trying to get permission from their parents to wed (one of them was a couple of months too young, under the laws of the time the real life event occurred).
In court, her mother finally admitted to having had an affair with the young man's father - so he was her half brother. They couldn't marry. Girl came unglued in her chair - they had deliberately gotten pregnant to force the permission.............
Her mother stood up and told the court that there was no problem, as when she'd found out that her husband was having an affair with his friend's wife & gotten her pregnant, she'd gone out and had an affair with his best friend and gotten pregnant, too. The young couple were much relieved, the wedding was performed by the judge, and the grandparents were left to work out the issues with their spouses having found out about the affairs some 17 to 20 years earlier (as I said, they were recreating cases with actors - I don't know what the age of majority was when the real events occurred that were being acted out for the tv show).
They had some really good actors and some of those court cases were more dramatic than any Perry Mason show!