Remember those play structures when you were a kid, with the swings and bridges and little doodads to play on? Like this one?
I think it would be awesome to design one the size of a small city. It would be eight stories tall, cover an acre, have millions of little doodads to play on, and could contain the playful energy of a million children. Parents from around the city or even state area would take their kids to it to have them play themselves to exhaustion, to be followed by a quiet nap time at home in which the parents work gets done. (Like that novel you wanted to write.)
Safety is a concern by the second story. (A typical "floor" or "story" is every ten feet in most buildings.) A human can usually endure a fall of ten feet okay, but a fall of 20 feet usually breaks a leg, 30 feet breaks both legs and the hips, and 40 feet squashes the unfortunate victim into "street pizza." The higher levels will have to be fenced, but that shouldn't be a problem for the designers. I once played on a structure four stories tall, and the safety precautions there were enough to keep all children entirely inside the structure.
Safety wise I would also want to keep adults out. We can trust a child's parents, but there are other adults that should not have access to the children. Probably a nearby police station to discourage that line of thinking is the correct solution. A park this large may already have one.
The best location for this site is probably a park in a large city, where there is a big demand for family activites for large numbers of people.