First, the $1400 round 3 stimulus is not anywhere on the tax return. Line 30 would claim the round 1 ($1200) and round 2 ($600) rebate if the taxpayer was eligible. Round 3 is paid separately by the IRS.
Second, if a person qualifies to be claimed as a dependent on their return, they must answer "yes, I can be claimed as a dependent", even if the person who could claim them agrees not to claim them. Answering "yes" will disqualify them from round 1 and round 2 recovery rebate on line 30 and they would not get round 3 either.
However, if a person answers "no, I can't be claimed" so that they get the rebates, when they should have answered "yes", that is potentially tax fraud. We don't know how aggressively the IRS will pursue this, but they have 3 years to audit all returns and 6 years in case of deliberate fraud.
Your best legal situation is for your child to correctly answer "yes, I can be claimed as a dependent" and they will get no rebate. Then, if you actually claim them as a dependent, you will get a round 3 payment of $1400 for them, even though you did not get a round 1 or round 2 payment, because the rules changed for round 3.
