i work at a restaurant, and the problem with not counting infants as part of the party comes with setting up tables.
coming up: we have a Christmas buffet. usually big parties. groups of 6, 8 12, or even 20 or 22.!
so when we set up the tables we set it up for that many spaces. so, we set up a table for, say 18, then the party comes in and says, "well there are 18 of us, AND we need 2 high chairs. (even if you turn the high chair upside down, to set the infant seat IN it) and they think we can just "pull up" 2 high chairs. where ? in the aisle?
so NOW, we find out, you REALLY needed a table for 20 (even if the 2 little ones aren't eating anything at all but breast milk, which is fine, they still need the physical space). and the table is all set up for 18, but there is NO room to bring in 2 high chairs, and the nearest 2 top is already seated.
this is why "understating" the number in your party by not including little ones who won't be ordering a meal(to avoid the automatic tip) doesn't work. .
most people do not HOLD the baby THROUGHOUT the meal. (nor should they have to! I know I wouldn't enjoy my meal if I had to hold the baby the whole time) most high chairs are made to turn upside down, to set the infant chair in it, so when baby falls asleep (or is content), mom, too.!(or dad, but we all know it's usually mom!

) can enjoy the dinner....
but that takes up space. you can't just place it "in the aisle". ( I need to walk through the aisle!) this is why Disney (or any restaurant) needs to know the total number of people, including infants, in your party.
as to whether this puts you at the "6 person or more" for the auto gratuity...
if it really bothers you, look at it this way... you are still taking up that amount of space in the server's station. it's just that your baby is taking up the space that a "paying customer" would have. 2 babies in one party, who are not eating, take up another 2-top, that the server could have in their station.
so: 2 couples , each with 2 little ones,, will take up 8spots. but, their bill will be less than the bill for 2 tables of 4. so, to protect the server, since her places in her station are taken, they add the 18% gratuity.(because a big portion of her station is taken up).
IF there is a problem with the server.. ie, bad service, rude server, etc,. where you felt 18% is not deserved,you are always free to bring that to the attention of the restaurant manager.
(all of my years as in the restaurant business, if someone felt the service was unworthy of 18% tip, I REALLY want to know that, and want to know the reason WHY, so it can be corrected).