You should definitely take them to the California Science Center near downtown LA. It's currently housing the Space Shuttle Endeavor and is definitely worth a visit. Plus, the museum is always free! You may need to make a reservation to see Endeavor, so go online and plan ahead. You can't just show up and automatically get to see it. http://www.californiasciencecenter.org/
They may also enjoy the Natural History Museum, which happens to be located right across the street from the Science Center. http://www.nhm.org/site/
If you want to stay closer to Anaheim, check out the Discovery Science Center in Santa Ana, about 10 minutes away. http://www.discoverycube.org/ The current special exhibit there is "Animal Grossology," which may be particularly appealing to boys. :D
If they are sports fans, you should see if the Anaheim Ducks hockey team is in town and go watch a game. You could also try for tickets to see the Lakers, Clippers or UCLA basketball.
Umami Burger is a popular restaurant with multiple locations and lots of interesting, very tasty burgers to try. 800 Degrees, owned by the same people, is a very popular build-your-own-pizza shop in Westwood (if you go to Westwood, you must also check out Diddy Riese cookies). http://www.umami.com/800-degrees/
Bruxie features waffle sandwiches. I haven't been yet, but people say it's excellent. From Anaheim, you'd probably want to go to the Old Towne Orange location. http://bruxie.com/ Old Towne Orange can also be a fun place just to walk around. If you're not too full from the sandwiches, there is a really good cupcake shop there, but I can't remember the name.
Speaking of cupcakes, you may hear that Sprinkles cupcakes are so wonderful but I must say that Susie Cakes (www.susiecakesla.com) are WAY better!! They are less known, but so delicious. Newport Beach is closest to Anaheim; there is also a location in Brentwood if you find yourself in Westwood/West LA/Santa Monica/Beverly Hills area.
Another thing to consider if you find yourself in Westwood is to see a movie at the Village (aka Fox) theater, where many movies have their red carpet premieres.
Aside from the last couple of days, it's been really warm here (75-85 degrees), so you might even be able to hit the beach. Newport has the Balboa Fun Zone, though I haven't been in 20+ years so I don't know what it's like these days, especially in February.