I'm not sure what mod -if any- this might be caused by, but I noticed my baby hanging out in the void, walking out from the bottom room of the house. I was able to follow them, and walk all around my house in fact. I was also able to access the staircase to the cellar from it, oddly enough...
Running 1.5.3, PC, Steam, English, single-player, no mods though SMAPI is running.
I've found a bug thread about an issue like this on Ginger Island Farm, but this is happening to me on the *home* farm (I chose the new beach farm). I wasn't able to find any threads about this one.
Forage that...