When I spoke to FS San Francisco directly with exactly the same problem as the OP they told me that they have plenty of rooms with 2 beds but they are single rather than double beds. This makes it impossible to fit in a family of 4 unless the children are babies or toddlers. My impression was that they were very child friendly - indeed they tried very hard to accommodate my family. I have stayed at several FS hotels and they have always made my children feel welcome.