With really little kids, I think so. As long as you're not hellbent on seeing everything, just the highlights. I would stick with Disneyland and skip California Adventure too. There's more for your kids there.
If you are just doing Disneyland with little kids, yes. Two if you want to go to CA Adventure. Try to get there right before it opens and go to Fantasyland rides first, like Dumbo. I tell the kids to pick the top 3 things they want to do, we do them first, then we wing it the rest of the day and don't spend hours waiting in line.
It depends on what you want to do and how crowded the parks will be. I think it's nice to be able to spread out the touring vs feeling like you have to do it all in one day.
I disagree on Disneyland versus California Adventure. We spent a lot more time at California Adventure than Disneyland.
Thor LOVED LOVED LOVED walking through Carsland even if he hated the rides he could ride there. He loved the (boring) Bugs Life caterpillar ride and loved Little Mermaid. He loved the Toy Story Mania ride. Most of his interests, Cars, Bug's Life, and Toy Story, were at California Adventure.
He also loved the Pixar parade at 4:30.
I agree with all of this. Plus the Monsters Inc ride!
We went for one day (my kids were 1, and my nephews were 6) and we had a great time. We didn't see everything by a long shot, but we saw everything that was important to us.
I think one day is enough for me personally. But if you have small kids, it's definitely fine. They won't know what they are missing if you don't return for a second day.