Kim,
It works best (and is easiest) if you put the note in the rese. when you are originally making the rese. on line.
Since you are past that, I would email the hotel with a small note "Celebrating daughters 11th B-Day on Friday". Not sure if this particular property is a kid friendly ES or more business minded.
They may come up with a nice surprise, but remember, it is a bonus if the hotel recognizes you, not something to be expected.