Yeah - that’s part of the “bug” that causes Hotwire to charge for kids when they shouldn’t. Disney only counts 18+ as adults in the room.
Interesting find on the different numbers of people causing different price changes. The only thing I’ve tested in the past was adding a single adult or child and saw the price go up; I haven’t played with it since.
In any case, I don’t think the number in the party will push the room to be King vs Queen often, if at all. And, if you plan to add people after booking with just 2, they wouldn’t likely lock you into a King - I don’t think the actual room assignment happens until some time later.
If it were me: I’d book the rooms with 2 adults and add the extra adult and kids later and see what they say about extra charges. I’d expect they will charge for the 3rd adult, but would not expect extra charges for the kids/teens.
Full disclosure: My philosophy is the same with Priceline Express, BUT Priceline specifically only guarantees room for 2. So, technically they could say “Sorry, only two people!” - But the vast majority of rooms at Disney resorts accommodate 4 and I haven’t heard of anyone having any issues beyond needing phone call or two to have 4 in a room.