Coming from Australia to drive from LA to San Fran in Sept.over 5-6 day period.Then staying at San Fran for 5?days, then on to LV through Big Sur and Death Valley.
Booking accomm. should we stick with one hotel chain eg. Best Western so as to recieve any benefits and know prices/conditions(hoping to not have to spend more than $100US a night) or take chances of finding acc. along the way.Will have a 12 and 14 year olds with us.(pool a must)
Thanks in advance for any help.

Since you have a specific budget and requirements, and are going to places that are very well traveled, I would book accomodations ahead. You might not be shut out completely if you don't, but you might have to do without the pool, or go well above your limit.

Check Best Western (or whoever else) to see if you would get any benefits from staying with them in multiple places.
But there won't be any benefit in terms of knowing prices and conditions. Prices will be different in each one (chain hotels like BW are often franchises that are individually owned). There also shouldn't be much difference in terms/conditions between all/any of the hotel chains.

September 3 is a national holiday.

Try to avoid spending Fri Sat evenings in Las Vegas and Fri Sat evenings on the California coast if you want to remain near that budget.
The chain hotels are not very well located in most of these places so I would go independent. Although on weeknights you can take chances that you will find accomodations, I would bring a list of recommended places and maps so that you do not spend too much of your time looking for a place to stay. You certainly don't want to stop at 10 different places until you find one with a pool big enough for a 14 year old, so do some research beforehand.

LA to San Francisco via the coast (SR 1, Hearst Castle etc.) can be easily done with one or two overnights, so if you want to take 5, pick some scenic area (I'd recommend somewhere on the coast near San Simeon or Big Sur) and plan on a couple of nights there, rather than breaking camp every morning for what would be a short drive.

Otherwise, take say 3 nights to get to SF, then shoot through and continue up the coast to Eureka (around 5 hours north of SF) to include the coast redwoods in your itinerary. Spend one or two nights on the north coast, then head back to SF before continuing south again.

You won't miss that much on the southern/central coast by limiting it to 3 or 4 days instead of 5 or 6, and by adding the redwoods you'll include some experiences that are unique in the world to northern California. Trust me on this.

If you're going to be there on Labor Day weekend (Sept 1,2, 3) you 'd be very lucky to find any place to stay without reservations. Even after that it's best to have reservations made, at least a day or two in advance.

If you're driving from LA to San Francisco over 5 days, you should go through Big Sur then, not on the way to Las Vegas. Then you could go to Las Vegas through Yosemite and down Hwy 395, far more scenic than the route through Bakersfield on Hwy 5 or 99.

Best Western isn't a chain. It's a group of individually-owned hotels.

Last summer we stayed overnight at the Best Western Cavalier Resort in San Simeon. It's very handy for Hearst Castle and is very nice, but there's nowhere to go nearby in the evening. They have a couple of small pools and the hotel is right on the bluffs, next to the ocean. The beach is stony, but there were surfers out. There is a huge grassy area on the bluffs where they have bonfires every night. The restaurant is full-service and the food is good.