Can't blame your DD for taking a little longer to catch on. I believe that the idea was for your DS to read the words on the cookie out loud. But he stopped when he understood the message.
I'll attempt to describe it for the connectivity-challenged:
Kids are called to the table, grandparents are milling around (with cameras), mom takes the cookie out of the (cold) oven and explains they will have dessert first- grandpa says it is a special dessert.
DS reads the message out loud
"Surprise! We are going to Disney World" "What?" "Really?" "Really" "What?"
He finally says "Today", tries to explain to his sister. Family gives the details about the trip- bags are in car, staying in a condo, grandma is coming on Monday, flight is at 4 today.
At which point DD holds up her fingers and says "I'm 4!"- very cute!
DS pretty much in shock and disbelief. DD non-chalantly rides on her
scooter.
As BigTex describes, she gets it about 10 minutes later, without the glare of the video camera.