Enroll your child in the Six Flags "Read to Succeed" program. At the beginning of the school year ask your K-6 child's teacher whether he or she is doing the Six Flags Read to Succeed program. In this program, Six Flags offers free tickets to all children who read and record their reading for six hours or more. The teacher receives a free ticket for coordinating, and your kids get free tickets, currently a $35 value!
Enter every sweepstakes you can where Six Flags tickets are offered. At websites such as Online Sweepstakes, you can learn about promotional offers and which contests are giving away free tickets. Enter, and hopefully win free tickets to Six Flags! Be careful -- telemarketers buy lists from these sweepstakes operators, so if you choose this route, get ready for some sales calls.
Find "buy one adult, get one kid ticket free" programs offered through Six Flags partnerships with major soda and chip companies. These partnerships vary year to year, but you can frequently get free kids' tickets to Six Flags through these promotional offers.
Ask for them on FreeCycle or Craig's List. It can't hurt to ask! Look on the "free" sections as well--sometimes people have day-specific tickets and something happens and they can't go, so they give the tickets away for free. You could be the lucky beneficiary!