I think they charge by the car and by the person. I took a bunch of toddlers and little kids rafting. With all the car seats and shuttle and stuff, we ended up with more cars than usual, and it ended up being kind of a rip off. You can do the lower-upper-C near Dotsero for free too.
SimpleMind is right, they might give your kids a popcicle one minute, then fly off the handle the next minute because you didn't pay for your carry-on bags or something.