Carlsbad is home to the LEGOLAND California Resort, which includes LEGOLAND, a water park and Sea Life Aquarium.

LEGOLAND California -
Keep the park's location in mind when planning your trip. Carlsbad is typically an hour's drive from downtown San Diego; it's much more convenient to have a hotel in Carlsbad.
The park may not seem all that large, but kids may want to spend more than one day. Multi-park tickets are LEGOLAND theme park, LEGOLAND Water Park, and Sea Life Aquarium.
Varous sections of the park have water play areas, and kids may get completely soaked. Bring towels, swim suits, or extra change of clothes. Water play is seasonal (California is not tropical like Florida or Hawaii).
The Beginning - At the entrance are lockers (stash extra stuff), and strollers for rent. Also, pick up sandwiches and picnic supplies at The Market.
Miniland USA - Eight different geographical areas with LEGO replicas of famous landmarks and city scenes. Kids will recognize New York City skyscrapers and Central Park, the Golden Gate Bridge in San Francisco, Las Vegas strip, Hollywood, New Orleans steamboats, the Capitol and White House in Washington DC. Water features, miniature people, and moving vehicles create realistic scenes. Also kids have fun piloting the boats at the Miniland Marina.
Imagination Zone - Mindstorms and Build & Test, older kids can try out and test their own designs for robots. (Our kids could spend the whole day here.) There's also a room for toddlers to play with DUPLO, popular with the stroller set.
Fun Town has the always-popular driving schools, where kids can get behind the wheel. Toddlers under 5 can start at Junior Driving School; for kids 6 and up, Driving School is just the first step to their license.
Dino Island - Little kids can dig in the sand to discover fossils, and the Lego T-Rex and Triceratops are stellar.
Pirate Shores - Arriving at Pirate Shores, expect to get wet! The water play area is fabulous - water slides and a big pirate bucket that fills up, then tips over in a torrent. Take the Splash Battle ride or stand on the bridge above the ride, and the kids will be totally soaked (keep any cameras dry).
LEGOLAND Water Park - Make your own LEGO rafts and float down a lazy river, wade pool with water cannons, really long water slides and tube slide, water play areas just for toddlers. Bring your swimsuits, lockers, changing rooms, cabanas are available.
Sea Life Aquarium - In Sea Life Aquarium, check out marine life of the California coast. Watch sharks, stingrays, jellies, seahorses gliding through the water. Touch sea anemones and starfish in the touch pools, and don't miss the feedings of the giant Pacific octopus. Aquarium is best with younger kids.
Museum of Making Music - If anyone in your family is interested in music, stop into the Museum of Making Music. The museum has instruments, from 19th century organs to Big Band saxophones to electric guitars to digital keyboards. Try playing the digital piano, drums, guitar or electric violin.
The Flower Fields (Carlsbad Ranch) - North County is famous for its flower fields, bright swaths of color, as far as the eye can see. From March to May , visit the Flower Fields. There are wide paths to run around, or take a ride around the fields in an antique tractor (it's a big area), and. There are picnic tables and you can get strawberry sundaes (topped with local strawberries and whipped cream) or milkshakes.
Beaches - Both Carlsbad State Beach and Carlsbad City Beach are long sandy beaches, and kids can easily spend all day playing in the water. Easy stairway access to both beaches, grassy areas and picnic tables. Kick back and relax.
Carlsbad Lagoon - The Carlsbad Lagoon is the perfect place to kayak, go pedal boating and paddleboarding, or just have fun with those big aqua cycles (the water is calm and protected).
Family Hotels

Here's Travel for Kids list of family hotels, all styles and price ranges, convenient to fun things to do with kids in Carlsbad:

Carlsbad hotels
Carlsbad vacation rentals