Of those three, I think Palisades is the most family-friendly. But the other two are fine for kids also.
The Legoland private entrance from MarBrisa is shared with the Sheraton. The walk from Palisades to the main entrance of Legoland is a bit further, but then you get the main entrance benefits (photo ops, kids' excitement builds, you're near the lockers and rental strollers and the big store, etc). Also, if you have early entry access (a benefit of some types of passes) then you have to use the main entrance (technically it;s just off to the side of the main entrance, point is you can't use the private MarBrisa entrance).
Seapointe is closer to the beach and you can walk, but it's not a super short, easy walk. You have to cross highway 1 and the train tracks. They give you wagons to haul your stuff but still... Also that part of the beach is not my favorite. I like the part that's closer to the main area of town.
As far as activities go, we go to Carlsbad every year. We spend time at Legoland and time at the beach. We also usually drive up the coast a bit to visit things like the
tidepools in Laguna Beach (they're fantastic, some of the best in the world). If it's strawberry season we always pick strawberries. It's just fun. We often rent bikes and go for a ride around town. We spend a bit of time shopping or window shopping. And we get ice cream at
SubZero, lunch at
Veggie Grill. We also try to stay up late to see the
grunion because they don't live anywhere but the California/Mexico coast. And of course there's a movie night or two. We also usually go
whale watching one day during the Summer. So far we've seen three different types of whales, plenty of dolphins, sea lions, and lots of other sea creatures. We've done SUP, surfing, boogie boarding, and kayaking.
There's lots to do right there, you don't need to to drive to San Diego. If you do go to San Diego, you can take the train because that's kind of fun if you live somewhere that doesn't have many passenger trains. We sometimes take it down to see a ballgame. The stadium in San Diego is particularly kid-friendly.
I've been meaning to go to the
Botanic Gardens and the museum of making music. We will probably do that next Summer. We also plan to go out to
Catalina next Summer and spend a day or weekend there. We want to snorkel out there to see all the Garibaldi and other fish you can't easily spot elsewhere.