What benefit are you thinking of?
If your child qualifies to be claimed as a dependent, your child must check the box on their return that says "I can be claimed as a dependent" even if the person who could claim them does not want to. That will disqualify the child from a stimulus payment. If your child does not check that box to try and qualify for a payment, they are committing tax fraud.
There is a separate question on the child's tax return that asks "Will the person who can claim you as a dependent actually claim you this year?" This only relates to eligibility for the American Opportunity Credit and will only make a difference if the child has income from working and the parent's income is too high to qualify for the AOTC. Otherwise, checking this box does nothing.
If you don't want to claim your child as a dependent, delete them from your return. But that does not make them eligible for a stimulus check.
