S.S.
ETA: YES!! To In 'n Out burger!! Wish we had them here in Georgia!!
I've not been in SoCal for almost 3 years. I miss it.
Carlsbad is the coast. In June it can be anything and everything - foggy, hazy, and yucky. Temps can be in the mid 70's.
You will encounter a lot of tourists at that time. Many others will have the same thoughts as you!
Expect traffic like you've never seen in your life. You will want to fly into San Diego and not LAX. San Diego Airport (aka Lindbergh Field) is about 45 minutes south of LEGOLAND and LAX is about 90 minutes north and straight shot up Interstate 5. If you fly into LAX, you'll have to maneuver around the 405 and then the 5 south. What a mess.