they normally don't do ultrasounds until your 8 weeks, unless theres history of m/c, spotting, ect. around the 5,6,7 week mark you might not even be able to see a heartbeat. If your really set on telling at christmas, I would call your OB, say that you have some spotting and cramping, and your worrying, and you'd like to get an ultrasound to make sure everythings OK. Some OB's do ultrasounds in the office, some you have to get a slip and go somewhere else.
I really think waiting is the best thing though, there's a possibility you wont see a heartbeat, and the whole holiday you'll be worrying. You could always tell them after the holidays, and a little new year cheer. Maybe send them a card that says "happy new years" along with an ultrasound pic.