Technically you don't have to chose, you can buy a park-hopper. The parks are a stone's throw away, unlike in WDW, and you could totally go half-and-half. It is more expensive though.
If you have really young, small children I would say
Disneyland hands down, particularly if they like Princesses, Pirates, or Winnie the Pooh. If they are more into Pixar...probably Disney California Adventure. I would suggest you look at the website, pick your top ten attractions, then go to the park with the most. Another way to go is by which character are found in which park. Kind of depends on your priorities.
I personally think I'd go to Disneyland for my first trip if I only had one day. There's something special about going to the original park. It's got a touch extra magic.