The only unrealistic thing in the whole manga is that they have only 3 kids
If you read the manga again Tsuki and Fujio really hard getting it on at home.. Kiri just really noisy about it.. They have to do it at expensive Hotel just to have peaceful sex LOL..
And 3 kids in Japan is REALLY EXPENSIVE and already a lot.. Uzaki's family is not that rich so they probably just wanna keep their economy stable..
There's already a chapter where Yanagi is actually earn more than Fujio.. Their economy is barely hanging in there because they have two source of income..
When Hana or Kiri leave they'll probably have another children..