I'm planning a trip to the Bay area end of Aug / beginning of Sept (from the UK) and have a couple of questions: 1) When does summer vacation finish for kids? And are there public holidays around that time? Just want to go at the quietest/cheapest time! 2) What are the best sport climbing crags in the area for an low grade climber? Thanks!
School starts back up in the first week of September at the latest, which lines up with Labor Day weekend. If you can postpone till the second week of September, things should be quieter. You might even get better weather.
T. White is on point with the crowds and weather advice. As for areas to go to sport climb, which is best depends on the specifics about what would be exciting to you as well as how much time you are willing to spend to get to an area. While there are crags in or near the Bay Area, a heat wave that time of year could easily make everywhere besides the higher elevations in the Sierra Nevada unpleasant and that is the time of year many climbers look forward to for prime Sierra conditions and weather.
This link will allow you to research and sort areas--