I think Disney ought to be clearer about this. Because children under 3 aren't counted on the dining plan, for park tickets or on room reservations, a lot of guests believe they don't count for restaurant reservations either.
Restaurants are different. It's not how many people are actually eating anything, it's how many are physically present. They need an exact count for capacity for fire codes, as well as space for high chairs. This is why when you show up with an extra person, no matter how old they are, and the restaurant is already at capacity, you won't be allowed in with the extra person. By law, they can't exceed fire codes by letting in one more person.
I don't think they get full to capacity all that often, and usually they'll work with you to get a bigger table if they're not - but you may end up waiting longer than you anticipated when you made the ADR, because you are a different party size now.