If you're spending 12-14 days and have never been to Northern California before, I'd suggest something like the following itinerary:
* 3 days in San Francisco. It's one of the most beautiful and eclectic cities in North America and there is a ton of things to do. My favorites include the ferry trip and tour of Alcatraz, riding the Cable Cars (including a visit to the Cable Car museum), having a crab cocktail at Fisherman's Wharf, visiting the Conservatory in Golden Gate Park, walking across the Golden Gate Bridge, and touring among the ancient redwoods at Muir Woods (actually about a 20-minute drive north of San Francisco)
* 1 day in Napa or Sonoma Valley. Great wineries and spectacular scenery. Some of the wineries have winery tours which even your kids will find interesting.
* 2 days on the Monterey Peninsula. Be sure to visit the spectacular Monterey Aquarium, have lunch in Pacific Grove, drive down one of the most beautiful highways in the world--the Pacific Coast Highway--to Big Sur, and spend time walking in the quaint and lovely hamlet of Carmel.
* If your focal point for your trip is Yosemite, spending a week there will be wonderful. It is one of the great awe-inspiring sites in all of the American West. Be sure to plan day hikes to the top of Nevada and Vernal Falls, drive to Glacier Point, have lunch in the beautiful dining room of the Ahwahnee Hotel, and go fishing on the Merced River. Be aware that the Yosemite Valley can be quite crowded in the summer (in fact, I can say with certainty that it WILL be quite crowded in the summer), so plan also to spend time away from the Valley by visiting the Mariposa Grove where you'll see some of the biggest and oldest trees in the world. You can also stay at the Wawona Hotel, which is on the western perimeter of the park--away from the crowds--and at a much cheaper price than other options in the Valley.
Have a great time! I am sure your vacation will be unforgettable.