The second stimulus is allowing any child that turned 17 at the last half of the year, will be eligible to receive the stimulus money. You do not have to return any money, you keep it.
From the IRS:
The IRS will use people's 2019 tax returns to determine their stimulus payments, which means that teens who hit their 17th birthday in the second half of 2020 — after tax returns were due to the IRS — could still qualify. No adult dependents will qualify for the $600 checks, according to the Tax Foundation.
