We use to live in the area. Legoland is designed for younger children, but when we took our 2.5 there weren't too many rides that she was interested in. She did enjoy the lego play area and the various things and towns built out of Lego's through out the park. With a young child I would not plan on more than one day.
Also in the area is the San Diego Zoo in Balboa Park near downtown San Diego. And the San Diego Wild Animal Park in Escondido. Both are world class facilities and in my opinion should not be missed.
You also have the beaches and marinas, and can do a harbor cruise. In San Diego we would take our young child to tour the Navy ships and she loves that.
It will possibly be cool and foggy in April. As far as traveling in CA I am not sure what you are asking. Traffic is heavy and congested and so have patience and plan for it. Freeways (called highways in TX) are plentiful and you can get just about anywhere via the Freeway system. There are more Toll Roads popping up but none in that area the last we were aware. Keep in mind that tolls in CA are about 3x what they are here and 10x what they are in NY.
Have fun, I'm jealous, we won't be back there until August.