Sequoia National Park
is about a 3.5 hour drive out of L.A., about 4 hours and some change out of Anaheim. If natural wonders float your boat like they do mine, then the drive is well worth it.
Sounds like a spring break getaway, but you said "a few days". To be honest, you really need to spend at least a week, no less than 6 days in SoCal just to scratch the surface. It looks like you two are coming from the East coast, if you're gonna make that trip for only a few days it would be frustrating. Disney World (much closer to you) is a better Disney experience and Universal is a total rip off + there's a Universal in Orlando as well. If you gotta do Universal, only do the Tram tour and leave.
My SoCal itinerary would be:
Day 1) Disneyland/California adventure (if you just gotta see the original)
Day 2) Hollywood Blvd + Venice beach + Santa Monica Pier
Day 3 - 4) Sequoia National Park -Return 5th Day morning
[optional] Day 3 - 4) San Diego Wild Animal Park + Sea World -Return 5th Day morning.
Day 5) Whale watching boat to Catalina Island
(an all day affair but is the best kept secret in SoCal)
Day 6) Chill out with Junior at La Jolla beach Kayaking and snorkeling around the Cove. Head back home tired as all get out that evening...but with 1,000 pictures that will last a lifetime.
If you could swing 2 weeks, I would have you renting an RV and driving over to the Grand Canyon (which should be a must on everyone's bucket list) for a few days and back through Hoover Dam, then
to Joshua Tree. You son's only going to be 13 once, and he's about at the age where hangin' with Dad is still fun.
Capture it now, even if it cost a few more $$ and he has to miss a couple days of school. It's priceless.