I married my gf (she's the farmhand) on our game and we just got the question/event for the third baby, the first went to the main house and the second to her cabin. I thought this was a glitch, anyways it doesn't have any sense to have babies on different houses.
This is a strategic move which keeps the babies from outnumbering you in any one place. If not, they'd force you to do their nefarious bidding, like making yummy Blueberry Jelly for them instead of filthy moneymaking Starfruit Jelly. Or digging up Strange and Ancient Dolls for them when you have a full backpack, and requiring you to toss your Prismatic Shards and Ancient Fruit Wine to fit them in.
Or worse yet: Convert the Wine Cellar to a rumpus room!
The children must be kept separate for
their our own good. Better yet, set up a Slime Farm in the house, now they have play friends!
...where'd those doves come from.....?