If you're staying near Disneyland, that means you'll be down in Orange County, which is a bit of a schlep from Los Angeles proper and its environs. Unless you've really got your heart set on seeing Los Angeles and the areas immediately around it, my recommendation would be to spend most of your time at Disneyland and places in OC that are closer to your hotel. Otherwise, you'll be spending too much time on the freeways in traffic crawling up to LA and then back again to OC. You can carve out a day for going up to LA and checking out some of the sights up there, but if your plan is to make multiple trips back and forth between OC and LA, you're going to be spending a lot of time in the car sitting on freeways.
Other posters have mentioned some good ideas for when you're in the LA area. I'm not as familiar with OC as I am with Los Angeles simply because I don't spend much time down there. That said . . .
You're already hitting one amusement park (Disney), so you may not want to bother with another one, but FWIW my kid liked the rides at Knott's Berry Farm better than the ones at Disneyland when he was around the same age as your kids. KBF is down in Orange County. The lines tend to be shorter at KBF, too.
You'll be near some good beaches, which are always a fun time for kids. Though it's been a long time since I hit an OC beach, Balboa peninsula (Newport) and Huntington Beach were fun back in the day, and I assume they still are. Corona del Mar State Beach and Laguna Beach are supposed to be good as well. If the weather cooperates, do a beach day, rent some bikes and/or boogie boards, etc. The kids will have a blast.
There's the Aquarium of the Pacific and the Queen Mary in Long Beach, which is closer to where you'll be staying than LA, Hollywood, Santa Monica, etc.