I was one adult three kids, so it was the same number of people staying at the site. I still had to pay the fee for the extra child.
I only had one child and often paid the rate for a family of four. I never asked for a discount-that was the going rate, period. I could take it or leave it. Too many other more important things to worry about than a potential $5 discount for having one less kid. However, I do agree that if the amenities are not available then you shouldn't be charged for them.
Oh, and put me in the camp with the others who think that it was ridiculous to make this post and not tell us where. Why are you hiding that?