Yes, its quite important that you include the infant. I read on the DIS Restaurant forum that there have been cases where parties have been unable to be seated because they hadnt stated the infants. The baby will still need a "place setting", be it high chair or booster, so has to be included in any reservation. Quite often under 3s are given their own special food, like Mickey Pancakes at Cinderellas Royal Table for example.
I would imagine if the restaurant wasnt busy then it wouldnt be an issue, but its best to make sure you include everyone!!! As Tracey said, the CM will ask the ages of the children and you wont be charged for under 3s whatever they eat (we never have been anyway!!).
Have fun!
Allie