Keep in mind that on a few rides, however, each 40-inch child will need an adult to ride with him. Legoland works best with a one-to-one adult/child ratio for kids under eight. I've done Legoland several times over the years alone with my two kids (now eight and five), but we have gotten more done on the visits when my wife is there with us.
One day should be plenty to see and do everything in the park, provided you arrive early. I've enjoyed a two-day visit there, though, breaking the Legoland stuff into two days and throwing in some pool and beach time each day, too. But one could just as easily do Legoland on one day, then the swimming the next.
If you want to experience San Diego and Carlsbad, then take 2 days.
Also, depending on how much time your four-year-old(s) might spend in the build-and-test area, two days might make sense. My Lego-addicted kid can easily spend an hour or two in this one area.
We just got back from a short trip to California which consisted of two days at Legoland and two days with relatives. Our children just turned 9 and 10 and they wanted to go back to Legoland for a third day but we just didn't have time.
We arrived at Legoland on the first day at 11:00 and left at 6:00. The first 2+ hours was spent at the new Sealife Acquarium which I would highly recommend. It is very new, very nice, very clean and very very kid friendly. Don't hurry through it - look and enjoy everything. Then we had lunch, went on some rides, took lots of pictures of all the Lego animals, people, let the kids play in different areas and ended up at the Pirate Shores water area later in the afternoon when it got really hot outside. We spent a good 2 hours in that area. The lines for the water ships and log ride were too long so we went back to those the following day earlier in the day.
The second day at Legoland we arrived at 10:00 when it opened and left at 4:00 (only because we were supposed to be meeting relatives for dinner, otherwise we would have stayed until it closed).
There is definitely enough to do at Legoland to keep you busy for two days, especially if you want to see everything and take your time and don't rush the kids (let them play as long as they want at the areas they want to play at - for instance, the Pirates Shores water park area we spent almost 2 hours at and the place where you build your own Lego cars and race them we spent another 2 hours at and the Hidden Hideaway play area and the Explore Village area we spent a lot of time at).
I would highly recommend two days so that you can take your time and enjoy as much as you can. Like I said, we didn't even see everything in two days and we will definitely be going back as soon as we can to see and do the things we didn't have time for this year.
Enjoy the time with the kids and family and take lots of pictures !!!!!
Walt Disney World's Magic Kingdom
Disney's Animal Kingdom
Disney's Hollywood Studios
Disney California Adventure
Universal's Islands of Adventure
Universal Studios Florida
Universal Studios Hollywood