FlitBen

I've done these except for the Grand Canyon/Death Valley stuff, but not all at once. But if you're up to it and have a car for a month, here's how you could do it, assuming you are departing from SFO at the end.

On arrival you'll want at least a week around San Francisco. With kids that age they'll need at least that long to adjust to jetlag, they can rest while you walk Union Square and the Ferry building. If you plan to visit the Exploratorium pier (and you should), get the International yearly family membership as your kids will be begging to go back many times. You can easily drive around Golden Gate park and the Marina area near the famous bridge, but if you're driving across to the Marin country side this early you'll want to register your car's plates and pay the online fee on the Golden Gate Bridge's toll website. Otherwise you will have to detour to a toll booth before crossing.

It's likely too early in the year as TWA884 said, but otherwise Yosemite is best as an overnight stay coming from the SF Bay Area. Especially if you are lucky to book a room in the park's Yosemite Lodge many months ahead. It's at least four hours by car from the city. Some say the chain hotels at Oakhurst an hour from the the park's south exit are fine. The central valley itself takes about an hour to drive to/from the main exits, so factor that. The good thing is there is a modern food court and supermarket in the park's village center, so you don't have to bring in so much food. I haven't tried this plan as I've only done a few day trips. From the south exit it's a 6-7 hour drive with stopovers down the I-5, to Disneyland Anaheim.

Disneyland and Universal plus Santa Monica and the beach cities with kids your age would need at least five active days to do right, so another week. After a rest day, the drive to Las Vegas will consume almost a day if you guys are passing by Cabazon outlet shops. If it's your first time, you'll want to stay least 2 nights on the Strip's center, near the Bellagio and Paris casino-hotels if possible. Other than the night sights there's nothing much for your kids but shows like the Knights chivalry thing at the Excalibur hotel-casino.

Brice/Zion/Grand Canyon is at least a 2 or 3-day from Las Vegas, plus one last night on the Strip to rest. That's about 3 weeks already. You'd have seen a lot of hot desert, and Death Valley might not seem so exciting at this point. I think you'd be better off driving south to spend 2-3 days at the San Diego Wild Animal park plus maybe Lego park at Carlsbad, all of which your kids will surely like.

Alternately, you could spend your last week driving north from L.A. on the coastal PCH past Simi Valley and Malibu, stay a night in Santa Barbara and visit their nice zoo and pierside, another night in San Luis Obispo to visit the vineyards and seaside towns like Avila Beach.

Then head up inland on the US101 and maybe stay a night in Marina or Palm Grove, to visit the Monterey Bay/ Santa Cruz for the aquarium and steam trains and redwoods. When you get back to San Francisco, after a rest day you could do easy day trips to nearby Coyote Point park or Marin headlands or Mt. Tamalpais/Muir Woods for a restful end to your CA vacation.

This itinerary will fit in four weeks as Moderator2 mentioned, but just about. You may be up to the drive, but will your kids?