Update
Ok so we called over there today and they said ANY ticket can be upgraded.
So basically we pay $21/person on Monday using our HSing coupon/e-mail they sent us and then for another $15/person we get in the 2nd day!

Considering that a 1 day kids ticket is $57 that's a really fantastic deal!
Mere: Legoland is sooo much fun, esp if your kids are 3-12. The last time we went my ds was about 3.5 yrs old and there were a ton of rides for him and honestly I had fun doing them with him. It was really enjoyable. Once they're about 36 inches they can do just about everything in the park and I think once they're something like 42 inches they can do absolutely everything. My 3 yr old is pretty small, she's 36 inches with shoes, and she can even do one of the roller coasters called the coastersaurus.
Also, if anyone's kiddos have food allergies they have a chart on their website with all their food offerings and what allergens they contain. Dd and dh both have celiacs and we were excited to learn they have GF hamburger and hot dog buns.