The reason why Universal and SeaWorld and Merlin are all building these parks is they are cheap to build and profitable. I can't believe it would be a 10 year timeframe to recoup investment. I have a feeling the percap must be ridiculously high with food and merch for kids.
I know from running a movie theater the concessions on kids movies blew away the "adult" movies with each kid needing their own soda, their own popcorn, their own candy, and most adults not wanting to deal with the hassles of sharing things which would have been far cheaper. The same kind of thing applies with all these parks-- high markups on food and merch and kids NEEDING that stuff. Disney knows it which is why they have been targeting 8 year old girls above all other guests.