Where do I enter a parent's AGI on a dependent child's return using turbo tax?

Other than acknowledging that they can be claimed as a dependent on someone else's return, a dependent's tax return has no connection to the parents' return. If the child filed previously, their prior year AGI may be required as part of the e-filing verification process, but the parents' AGI plays no role.

Thank you! Didn't realize this changed for 2018

For future people finding this, the rules did not change between 2018 and 2019. You must enter the parents AGI in specific cases. For example if your dependent earned more than $2,200 in unearned income (as of 2022). This could happen if the dependent had a stock account for one possibility. If you are required to enter the AGI, TurboTax will ask for it. You won't need to search for it.