@katebert164
You are never required to claim someone as a dependent. If you discover that you have a dependent that you didn't know about, the IRS doesn't care whether you claim that dependent or not.
Also, as DoninGA said, you can claim someone as a dependent on your tax return even if that dependent wasn't counted on your W-4.
The fact that someone is your child does not automatically make that person your dependent. There are a number or other requirements for someone to qualify as your dependent. The chart on page 11 of Publication 501, which VolvoGirl gave you a link to, is a good summary of the rules for whom you can claim as a dependent.
You might have a child that you don't know about, but you are not likely to have a dependent that you don't know about. To be your dependent the person would have to have either lived with you for more than half the year or been supported by you. Someone that you have no contact with at all is not going to meet the requirements to be your dependent.
As far as someone being deceased, they could still be your dependent for the year that they died. Someone who died during the year is treated as having lived with you for more than half the year if they lived with you for more than half the time they were alive during the year. But again, it can't be someone you have no contact with. They have to have either lived with you or been supported by you.
And again, if you somehow discover a dependent that you didn't know about, you are not required to claim that person as a dependent on either your W-4 or your tax return.
