In my experience the babies are "usually" too small to clip through the geometry and spawn outside. This seems to happen when you enter the area and the game loads in the objects. Seems that it loads the critters before it loads the walls. Double wall or double floor may (or may not) solve the issue. This is one reason why I build my barns on solid ground and inside an earthen wall. They might pass through the wall of the barn but they won't also pass through the earthen wall before loading is complete.

Let me propose a hypothetical for you. "The ground is shaking" trolls throw rocks and destroy a section of your floor.......

Those floors only have 1,000 health per section. Even graylings and graydwarves throwing rocks will eventually break through and I don't see an arc underneath.