The Lego Kids Club magazines sometimes have coupons for Buy One Adult ticket, Get One Child ticket free. I believe there is one in the most recent magazine. However, you can purchase the deal online, and have the tickets emailed to you. The tickets are valid for one year from purchase date. Also, if you need to get multiple free tickets, you can. Just do them in separate transactions. You must go to this website. (The standard legoland website does not allow for promotional codes)
I just purchased 4 adults tickets and got 4 free tickets for children. (Had to do 4 transactions though) The emails come pretty quickly. Hope this helps someone out!