Laguna Beach is a beautiful location with lots of boutique stores and art galleries; also many good restaurants. Great if you want to enjoy beautiful views and shopping.
Huntington Beach is a great place for a family beach vacation. You can easily go swimming here; if you're lucky and get one of the fire circles have a hot dog roast; great for walking out on the Pier, etc. For kids age 7 and 10, they will like this area.
Newport Beach is between Laguna Beach and Huntington Beach. If you want to go shopping, people stay by Fashion Island, however its still a drive to the beach. If you want a beach vacation, people tend to stay on Balboa island. Its also where there is a small amusement park; ferry to Catalaina Island, etc. Its right by the bay and by the ocean. There are a variety of vacation rentals as well as motels/hotels in the area. Its a good place if you want to do vacation home rentals. We have also stayed at the Bay Shores
http://www.bay-shores.com/
Any of the 3 is really nice. As you know, its really important to have a car in the LA area. For Disneyland, if you are just going for a day or two, any of these locations work fine. If you are going for more, its easier to stay by Disneyland. Legoland is a bit of a drive - expect about 90 minutes to 2 hours to get there, but doable for a day trip.
For hotels on beach itself - Huntington Beach hotels are directly across from the beach (you do need to go across highway 1) but easy to do. The two to look at is the Hilton or Best Western, both good locations. If you want a walk way across Highway 1, then Hyatt Regency works well. Any of the 3 makes for a delightful vacation.
In terms of weather - March is not swimming weather as you know - but its fun. The only bad days are really rainy days or days where the wind is blowing (i.e. sand tends to pick up.). On the other days, its great as most of the locals will always tell you "its too cold - who would go to the beach in March.". Don't worry about that - we've enjoyed wonderful days at the beach without the summer crowds. There are always surfers and others to watch year round as well. Oh - wish I were there now.
Enjoy your vacation.