So basically all of her kids would stay free, and the people at the hotel would say 1 adult and 2 kids in each room.

Word of advice: I'd let the hotel think it was that way. Some get snotty about kids in a room alone. I don't know why, but some do. I worked for a major chain in college, and I know it was frowned upon. You have to make the front desk think there is an adult in each room. Since all of the kids will be under 18, that means the 1/2 in each room theory. It also could end up saving you some money since some hotels (especially the Hawaii ones for some reason) seem to have lower 1 person rates.