Seth and Alexa have a very beautiful guest apartment and are good hosts with responsive communication. The guest apartment is very clean & modern with beautiful finishes. We were a family of 4 (2 adults, 2 children) and stayed for 2 weeks. Although not explicitly "baby proof", the apartment served us well nevertheless. From the description we didn't realize the kitchenette lacks a stove, oven, freezer, and most kitchen utensils - problematic if you come with a family and rely on making meals for the kids. But, as Seth & Alexa do mention, the location is within walking distance of some of the best restaurants SF has to offer. Also lacking is a washer and dryer - but there are several wash&fold services within 1 a block radius.
The guest apartment is only separated by a staircase and a curtain from the rest of the townhouse. So when Seth & Alexa are entertaining, the noise travels down to the guest apartment. Likewise, any real noise created by us (crying kids), would have been overheard by them. So, although we had no specific problems, it is something to be aware of.
There is an iron gate separating the sidewalk from the apartment door, which is good considering a few homeless make other stoops in the neighborhood their overnight shelter. The apartment also has an alarm system for added security.
For those with kids: the floor is concrete and very unforgiving if a kid falls, especially off the bed. Didn't happen to us, but it was a concern.