We did San Fran/Yosemite/Zion/Grand Canyon/Big Sur and Santa Cruz/San Fran. That was spread over more than two weeks.
As someone who has done the twin thing, wouldn’t contemplate that with two 5yo, to be brutally honest. Yosemite is fantastic to look at, but, apart from a paddle in the Merced and a trot up to Bridal Veil Falls, don’t think they’d enjoy it all that much.
GC even more so. Nice views off the rim for you but the only other thing to do is trek into the canyon. There are virtually no facilities apart from camping/visitor centres, and none aimed at nippers.
Even SF I would have said was a more ‘grown up’ city, with plenty to enjoy as an adult, but it would be frustrating restrictive with two little ones in tow.
Plus the flights, obviously, which will be a challenge even if your kids are well-suited to flying.
We were considering a US trip with our two next year, but maybe we’ll leave it a couple more so they can get the best out of it. They will be 13 this month. 🙂
PS If you do go, stock up on all the decent food you’ll need while in SF. We were staggered at just how awful the food was outside the cities. Unless you can live off mega packs of twinkies, cheetos and beef jerky.