We are WDW vets, that also love DL. I would say you are missing out if you skip CA. Radiator Springs Racers is probably our favorite ride in any park on both coasts (it puts test track to shame). Carsland is quite incredible, especially at night. The view when you enter from the wharf took my breath away the first time. California Adventure has the Monster's Inc. ride that you won't find at WDW. They still have the Animation Academy, which you can no longer find at WDW. This summer TOT will have completed its conversion into a Guardians of the Galaxy ride, which from the description looks like it will be pretty impressive. World of Color is terrific (takes planning though since it is a standing only show). If you can only do 1 day, you might at least add a hopper to get in a ride at RSR-- with the kids ages you could do the single rider line which often doesn't have that long of a line. I think we rode RSR about 4 times in a row one afternoon last July with a wait between walk on and 10 minutes in the single rider line. Also, unless they change it before this summer, you can meet and greet Oswald at California Adventure.
Our first DL/CA trip was just DH and I on a business trip. We spent one day with hoppers, went at breakneck speed from open to close (counter service meals only- no sit down) and got a lot done. We fell in love with DL/CA that quick trip- which surprised me because I thought I was a total WDW girl- I had been to DL once as a kid and remember thinking I did not like it as much as WDW. Now, if you ask me which I like best, it is kind of like asking someone which of their kids they like best. With DD (10) we usually spend 3 full days and 2 half days, and still feel like we could spend more time. You can get a taste of DL/CA in 1 day, but it may not be enough for the kids- if you can swing 2 with all your other activities, I would try to fit another day in. Also, with a short time, I would invest in the hoppers. Unlike WDW at DL, the parks are separated by only a plaza, you don't have to go through bag check again when hopping, and it is literally 5 minutes to get out of one park and into the other.