I recommend buying them online to save money ($31 vs $37) and time.
You get free admission if it is your birthday.
A regular ticket for a child 3-12 is $37. This ticket is good for entry anytime during regular business hours. You can save 20% by entering before 11am or save 30% by entering after 4pm. If you are planning on visiting several of the Atlanta attractions I would highly recommend checking out the CitiPass. A child's CitiPass is $62. You can visit 5 attractions from a list of 7 for one price. (The pass is valid for 9 days so you don't have to rush to see everything)
Varies. Thier website states $40 for general admission. If you buy them online and early you can get them for $31.
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