You can add hoppers by themselves.
You can add hoppers + water parks.
But, for whatever reason you cannot add water parks w/o hoppers.
We have 4-day hopper + WPFM for our family of four. Would have been $1,339 full price. $754.xx through Y.E.S.
You can sign up the entire family, but you cannot have more adults (real adults, 18 & over) than kids with an exception for adults who only have 1 child. In our case, we have Dad, Mom, DD11, and DD6. Only DD11 is signed up for the class (Mom has to go with her), but all 4 of us have tickets through Y.E.S. Say we had another child who was 19; we'd have to buy a regular ticket for that child.