I went in December 07, the park was not busy at all. We used the fastpass for a couple rides, but most places it is not needed.
If you are going to be there for a week or more, then I strongly recommend getting the southern California city pass. 3 days at Disneyland/California Adventure park (it is a park hopper pass, and you get to go in 1 hour before the park opens to the public for one of those days), 1 day at Universal Studios, 1 day at Sea World, and 1 day at your choice of the San Diego Zoo or the Wild Animal Park.
http://www.citypass.com/city/socal.html
When I went there, it was between 15-25 C, and the locals were wearing scarves and mittens.
I went to grab a frozen yogurt, and outside the store they had these heaters you could sit under. It was like 18C...
The Fastpass is
free, everyone gets it.
DO NOT FORGET to visit the tiki hut. Scariest effing ride I ever went on. Your kids will be fine on it though.
Take the kids to see fantasmic one night, it is a really cool show they put on over water... and camp out a good spot for the fireworks somewhere near the Disney statue.
If the lines are really long at the california park, there is another entrance through the adjacent hotel. I never stayed there, but they never cared that I was using that entrance. No lines.