Sarah, congrats on your book! I think that the outfit you described sounds perfect. But as tarzy said, you can really wear anything you want. I have seen people doing book signings in jeans, some in shorts, and some in suits! You're actually in a great situation because there are really no expectations and you can wear whatever makes you feel great!
AS for leaving your daughter, I'm not a mom, but I know my sister-in-law recently had to go on a trip for work for an entire week, leaving her three year old and one year old at home. It happens, and it's part of having a career. The three year old, although attached to her mother, was totally unphased by her mom being gone. My sister-in-law did leave her a little note with a riddle that led her to a hiding place, where there was a special gift for her. My niece got a note from my brother every morning and then found her gift. Maybe you could do something similar?
As for SF, I'm sure you'll get good recommendations from others who live in the city, but I have visited a few times and absolutely adore it. It's a great city and you should have a lot of fun. Definitely don't be nervous about traveling alone--it's a great way to explore and see the things you want to! I would suggest going to tripadvisor.com and clicking on their forums for San Francisco. If you post questions you have you will get fantastic answers from locals.