Mumsnet Talk

we have 3 week in March/April and so far have

Disneyland / La for 6 nights

8 nights (? somewhere for 4 nights in Big Sur + 4 nights somewhere else)

7 nights in Yosemite - the start date of this is fixed as family meeting us.

5 nights - ?

Fly out of San Fran

Help! The youngest is 6 months.... we are trying to minimise travel...

I'm from SoCal & have been there with 3 under 6 (not 3 under 5, though, you will have pretty tough jet lag I fear!)
I would say
Days 1-3 Fly into LA & camp on the beach in daylight hours & try to get all onto local time
Days 4-6 Disneyland, I usually do it in 1 day, but guess you can stretch to 3+.
You will have long queues if 2 weeks either side of Easter (blech).
Days 7-12 Travel slowly north, some time in Big Sur, see Monterrey Aquarium. Look for Sea Otters. See the butterflies in Santa Barbara, maybe.
Days 13-14: SF things
Days 15-22: Yosemite.

that's 3 weeks, right?
If you have more time, could sandwich in a visit to San Diego attractions between LA & travelling north.

Nobody on MN but my family on ever has terrible jet lag, though, the others don't wake up at 3am for the first 5 days & have to crash out at 5-6pm, & need a week to get on local hours (both ways). & have kids up at weird hours in-between. No idea how they do that!

Make sure you spend the first night somewhere where the kids can do something at an impossibly early hour (we found a great place in Las Vegas where the pool opened at 6 am). Get lots of sunlight, to readjust the internal clock.
For the parents you could try the jet lag diet, definitely try to eat lots of fruit and veg, and little meat; part of the yucky jet lag feeling is undigested food in your stomach. Which is why it is noticeablefrom the East to West coast, 1 meal time difference.
I'd spend time in SF at the end, there is lots to do (a great hands on science place), you don't even need a car in SF.
Between LA and Yosemite, I would plan a route that takes in the coast and highway 1 as much as possible, and take it slowly. We did S to Yosemite in 1 day, the day after landing, when we went.

Actually on that trip we also went to one of my favourite places, which is Mendocino, but that is 4 hours north of SF. A book like "Hidden San Francisco and Northern California (Hidden San Francisco & Northern California)" might be able to give you some ideas.