What you claimed on your W-4, at work, was just an estimate, for
withholding purposes. It does not lock
you in to what you file at tax time. You don't enter it anywhere on your actual
tax return. The IRS does not try to match that to what you actually end up
filing for.
So, just file as Head of Household, with your dependent. If it turns out that you owe money, you should file a new W-4 at work. You are not exempt.
As someone else said, you are not technically "exempt" unless your calculated tax was 0 last year and you expect it to be 0 this year.
