What is the equivalent adult IQ of a 120 child IQ?

First, IQ tests are designed so that your score takes your age into account. This means that (ideally) a person exactly as smart as you, but older or younger than you should get the exact same score. (So if a person exactly as smart as you, but 30 years old took an IQ test, they should get 120, or someone 50 or someone 15). Though this does not always work out perfectly in reality.