If your son or daughter or other dependent files their own tax return and claims a personal exemption for themselves, you will not be able to claim them on your tax return.
If the value of the exemption is worth more on your personal tax return, as it often is, your dependent would need to amend their tax return and you will need to paper file your own tax return.
If you are in this situation, I can calculate the value of the exemption on each return for you.