Thanks so much for your advice so far. Southwest Dave, we are going to take your advice and travel in the reverse direction and do San Diego at the beginning, Anaheim at the end which I think will be good to split the theme parks up rather than having a 10 day hit.
Basically we are keeping the itinerary pretty much the same however with adding a couple of nights in and stealing a night from San Los Angeles we have some extra time that I would like to spend in the Utah/Arizona area.
So, I am thinking something like the following:
1nt Phoenix
1nt Sedona (mainly so we can take the kids to the Out of Africa Wildlife Park)
2nt Grand Canyon
2nt Page (need 2nts here as we want to do float trip and Antelope Canyon)
then 1nt Monument Valley, Bryce or Zion (we can pick two of these)

So out of Monument/Bryce/Zion which one would you drop? Or do we drop the Wildlife Park and do all 3? Keeping in mind we are travelling with 2 8yr olds would it be 'too much canyons and too much of the same' if we do all 3. Am thinking the kids will really love the wildlife park so that will keep them happy!

Also we have decided against staying in Death Valley and are have a look on the way through and stay probably in Bishop to put us a bit closer to Yosemite. And yes Yosemite is definitely on!

Any tips and suggestions would be really appreciated.

Thanks