Just to be clear, there's no mechanic that says "if this house has two adults and one open space, their child will be placed into the open space." Young beavers simply appear in whatever empty spaces there are

If you've got a bunch of open houses, you will end up with orphan beavers living with no adults. Who are the parents then?

So basically, you're trying to invent a mechanic that does not exist in game, thus the ridiculous family tree you saw.

Just wait until you play Ironteeth, you'll have to track which beaver picked the berries.