Honestly, what we've done in situations like that is make the reservation for the max number of people that would possibly be there, and then when we show up, we say "Sorry, these two are sick" or something. I know it's not the most honest thing, but when you call back to change, you always risk it getting accidentally cancelled altogether or something like that. Definitely DON'T make the reservation for just your family and assume you can call back and add 2 people.
ETA: The only time you would want to call later and cancel for the additional people is if you're eating at a restaurant that requires you to hold it with a credit card.