When you make your reservations, you'll be asked how many in the party, adults and children. They will also ask the ages of the children. You need to include your infant in the number of the party. At that time, you can request a high chair for the little one.
For counter service (quick service, fast food), there are high chairs in the seating area. Since you do have an infant, you can get your high chair and your table before one of you goes for the food. If, for some reason (I can't think of a single time this has happened), you cannot find a highchair, tell one of the Cast Members and they will find one for you.