I ususally advise first time visitors with children to avoid Magic Kingdom on their first day, anyway, because it usually is the children's favorite park and they might not fully get all that they can out of the other parks if they are thinking, "This isn't as good as Magic Kingdom." Or, "I wish we were at Magic Kingdom." With your family being
Disneyland veterans, that might not apply, though, as Magic Kingdom has so many similarities to Disneyland that the kids might already know what to expect.
If it were me, I'd look at starting out Monday at Epcot. Easywdw has it the only recommended park and Josh is usually spot-on with his recommendations. Despite it's reputation, there really is a lot that kids can enjoy here. My three year old daughter absolutely loved Soarin' in January and has loved Spaceship Earth since her first trip in December of 2012. I bet your son would also really enjoy Test Track. It's a similar concept to Radiator Springs Racers, but you actually get to design your car before you start the ride. I prefer RSR, but I know many who absolutely LOVE Test Track, and many of them have younger boys. Turtle Talk with Crush is fun for all ages and I would recommend it not be missed (then again, you know about it at DCA). The street acts in World Showcase are fun. Make sure you check out Sergio in Italy as he does a great job of getting the kids involved. In fact, with your daughter's age, she would most assuredly end up throwing a ball into his net. Also, don't forget about Kidcot stations and Agent P's Adventure to help keep World Showcase fresh and fun for the kids. For dinner, I'd suggest Garden Grill in The Land Pavilion. The food is excellent and fresh, served family style, and the kids get their own meals but are more than welcome to eat the adult foods, as well. Chip, Dale, Mickey, and Pluto come around and greet you and, unlike most character meals, will come to your table multiple times.
Tuesday, I'd head to Animal Kingdom. It's the second most recommended park on easywdw, but I'm going to save the number one recommended park, Hollywood Studios, for later in the week. Possibly look into a Tusker House breakfast to help you get into the park early. You will dine with Donald, Daisy, and Goofy. It's a buffet and it is located in Africa so when the park officially opens, you already have a head start to get to the safari, which I find the most fun early in the morning.
On Wednesday, I'd head over to Magic Kingdom. If you want to do Bibbidy Bobbidy Boutique with your daughter, this would be a great time. And, you could see about getting your son a Pirate's League reservation for the same time so they both have something fun.
On Thursday, I'd head over to DHS. This has turned out to be my daughter's favorite park which, I must admit, I love as it has been my favorite since I was 8. That said, it does seem a bit polarizing. Some kids love it and some just don't. If your son is at all into Star Wars (or your daughter for that matter), I highly recommend Jedi Training Academy. I'm not really into Star Wars and I love watching the kids. Make sure you sign up for this first thing, though, if you are interested as spots fill up fast. If your daughter still likes Disney Junior, I'd look into the Play and Dine at Hollywood and Vine. She will get to dine and dance with Sophia, Doc McStuffins, Handy Manny, and Jake. It's a fun one for the kids.
Friday, I'd head back to Magic Kingdom. If your kids really just loved Animal Kingdom and want to do that again since there is nothing like it at Disneyland, keep that option open, as well.
I hope this helps you out. If you have any questions or concerns, please don't hesitate to ask.