You might want to get in touch with Janae Noble @ Noble Pursuits. She set up several ladies only California rides - I knew here from the Princess Promenade, which eventually my club helped SAG for, and she was my cycling coach (I needed a little/lot descending confidence help). I think that first year was more than a little exhausting for her - permits, vendors, lining up sponsors... and the first year it was rocky. You definitely need help from multiple clubs - road painting, route guidance, SAG (get a high school cycling club for kid ride alongs/chaperones if you do have a youth option), sweeps, clean up, set up, adverts... The ride in Sacramento uses a multi-purpose recreational trail which was open to other cyclists/users at the event day, but that also limited permit/liability woes. There was an optional time trial, which was fun!, and costume contest. However, it seems to have really taken off. My daughter also participated in the ride, and she enjoyed it. We tough girls did not so much enjoy the princess aspect - but I did half-heartedly go as a Valkyrie.

I cannot much help but offer encouragement. While my ride buddies are now mostly men, so I tend to prefer mixed gender events, I think rides like these are encouraging in building confidence. That said, I love the pre-Mothers day idea. I did used to lead a women's only ride before moving to the SF Bay Area where I still have not joined a club - my ladies group added in some skill building workshops, which was helpful - and could be useful for getting the word out if you target clubs (e.g. cycling clubs, school clubs, Girl Scouts, 4-H). I might suggest you consider contacting your local hospitals/health plans as sponsors - or even the local council. Try the education department if you go for the hospitals - very often they have money for health related activities.