Under the law, if they qualify to be claimed as a dependent, they must answer “yes” to the question “can you be claimed as a dependent by someone else?” on their tax return, even if the person who could claim them agrees not to claim them. Answering yes to that question will disqualify them from any stimulus payment in their own name. Answering no, to get stimulus payments, when they actually could be claimed as dependents, will probably be considered tax fraud, although I don’t know how well the IRS will be able to enforce this. The IRS has three years to audit any tax return, or six years if there is an allegation of deliberate fraud.
