Baby checker

A virtual state that checks whether the user has added any babies. This is a very small feature.
Similar to the comment I made on the previous PR, I think it would be better if the tests all had a `.input()`, so that our tests are simulating how the user would reach this state.

I think its maybe a little too subtle (and we should think about fixing this), but if you don't specify a `.input()`, its basically the same as using `.start()`, which isn't the same way that the user would have reached this state.