Pricing structure is a matter for the provider of a service to set, and for users to accept or reject.
We don't know what the daily rate before or after awning cost was, so the cries of "rip off" are somewhat premature - if the site has an underlying pitch fee half that of anywhere else, and then adds bits for extras like awnings, what's the problem? It's the same as charging more for an EHU, or showers, or hardstanding, or for being a non-member, or even for twin axle (which has been known).
Easyjet and Ryanair have adopted the "pay only for what you use" model and it has very successfully reduced the cost of air travel for all consumers- it would be a bit silly to say it's a "rip off" to have to pay to put a case in the hold, or have a legroom seat, or priority boarding, or a meal, when it's half the cost of BA even once you've selected all the extras.
Only scenario I'd be annoyed by it is if you'd asked for a quote and been given a price, and only once you're there do all these extras come out of the wood work. If they gave you the price without asking about an awning then when you arrive start adding it, that's out of order.
The child rate debate is a bit bizarre. It's not about reflecting the cost associated with the child staying there, it's a marketing tool used by the site to entice/facilitate families to stay. They have to draw a line somewhere age-wise, otherwise you'd have smarty pants 70 yr olds with their 40yr old "children" taking the mickey trying to claim the discounted rate.