January (after New Year's weekend) is a great time to go as far as crowds go. (Except for Martin Luther King weekend, when the crowds wll pick up a bit.) Depending on when you are there, you *might* still be able to experience Haunted Mansion Holiday and "holiday it's a small world" - or they might be closed to remove the holiday decorations. Those usually stay up through the second or third weekend of January.
It IS off-season, though, so unless you're there on a Friday or Saturday night you won't see the fireworks or Fantasmic!, and the Electrical Parade won't be running at DCA. And there will probably be several attractions down for refurbishment.
That's also California's rainy season, so you might have rain during your visit. Of course it could also be 80 degrees - you never know.
"Must dos" with young kids...Fantasyland, especially Dumbo. Visiting Mickey and Minnie's houses in Toontown is fun, too - and you can meet both Mickey and Minnie there, also. If your girls like princesses, they might enjoy meeting the princesses outside the castle (they are usually doing character greetings there with Snow White, Aurora, and Cinderella), or meeting Ariel at King Triton's fountain. The Pooh ride in Critter Country is fun, too. The Buzz Lightyear attraction and Autopia (fastpasses available for both of those) are usually popular with younger guests, also. And if you're going over to DCA, do NOT miss the Aladdin show - it's really well done, and fun for all ages! There's also Playhouse Disney at DCA, and "a bug's land" has lots of attractions just for kids their age.
What can be missed...if your kids aren't into thrill rides, then you could miss "the mountains" - Space, Splash, Big Thunder, and Matterhorn, and the Indiana Jones attraction (though I'd hate for the adults to miss that one - it's the best ride at
Disneyland in my opinion!). And a lot of the rides at DCA will be out, like California Screamin' and Tower of Terror.
Laura