I go down to Carlsbad/Legoland quite a bit and usually can get to Legoland within an hour from Huntington Beach. It is just North of Newport Beach. You just have to make sure you don't drive during traffic hours. The latest I have left to get back home was 4:30 and I was quite surprised at the little amount of traffic there is. When it starts to get bad, there is a toll road that takes you through Newport Beach too. You can always stay in Carlsbad for dinner and leave after 7.
ETA: I am not sure when you are coming down, but if you are here before April 1st, I have Legoland coupons, buy 1 adult, get a child free. You can have them if you want them.