As others have said, there is no advantage for the kids claiming themselves. You would lose the $500 other dependent credit. The credit begins to phase out for taxpayers with income of $200,000 or more ($400,000 for married filing jointly.
With the tax law change, effective 2018, most students will get the same refund whether they claim themselves or not. The personal exemption has been eliminated and the standard deduction increased. However, you only qualify for an education credit or deduction, if you are not a dependent*. With $6000 of earned income, they are not even required to file a tax return, except to get any income tax withheld refunded.
Your income is too high to qualify for a tuition credit. Their income is too low (less than $12,950 of earned income) to qualify*.
But, all that is academic. It is not optional for them to claim themselves. If they can be claimed as your dependent, they must check that box on form 1040, whether you actually claim them or not.
*If the student actually has a tax liability, there is a provision to allow him to claim a non-refundable tuition credit. But then the parent must forgo claiming the student as a dependent, and the $500 other dependent credit. The student must still indicate that he can be claimed as a dependent, on his return. This is worth up to $2500 (AOTC shifts to all non refundable)
