This may be a slightly garbled recollection of R. A. Lafferty's 1967 short story "Ginny Wrapped in the Sun", first published in Galaxy Magazine, August 1967, available in the Luminist archive. You may have read it in one of these compilations; perhaps in Lafferty's 1970 collection Nine Hundred Grandmothers, which can be borrowed (for free but registration required) from the Internet Archive, or else in Robert Silverberg's 1974 anthology Mutants, also available for borrowing from the Internet Archive.
The title character, Ginny, is a four-year-old girl. She does demand a large number of peanut-butter sandwiches (no jelly), but there is no mention of anyone selling a house:
"All I asked of Mother is that she make me three thousand seven hundred and eighty peanut-butter sandwiches. Isn't that a fair request?"
"I'm not sure that it is, Ginny," Dr. Dismas said. "It would take you a long time to eat that many."
"Of course it will, twelve hundred and sixty days. But that makes only three a day for the time I have to stay hidden in my nest up in the rocks. I figured that out by myself without paper. A lot of kids that have been to school already can't figure as well as I can."
Agar Dismas and Dall Minden had sauntered down from Doolen's Mountain.
"We're going to get four hundred and seventy-three loaves of bread and four hundred and seventy-three jars of peanut butter," Agar said rather nervously.
"Yes, Ginny says to use Crispy-Crusty bread," Dall Minden detailed. "She says it has sixteen slices to a loaf, so we can make eight sandwiches to a loaf and to a jar. There will be four sandwiches left over, and Ginny says we can have them for our work. She's going to stay in her cave for twelve hundred and sixty days. She says it will take that long to get her thing going good so nobody can bust it up. I think she's a numerologist at heart. This is going to take more than four hundred dollars. That's more than Agar and myself have saved up together. Ginny says to do it, though, even if we have to steal the money for it. And she says to be quick about it."
"Why aren't my servants back with my provisions? They've had almost time to get back if they did everything at breakneck speed and had good luck. They might have had to go to more than one place to get that much bread and peanut butter, though. I doubt if I'll eat it. I just want to have it if I need it, and I wanted to teach them obedience. I'll probably start to eat meadow mice and ground squirrels tomorrow."
There is nothing about a theme park, but here is what the story says about children, monkeys, and people:
"She is in good voice," Dr. Minden said. "When she speaks of her servants, she means your daughter Agar and my son Dall. It scares me, for I almost know what she means. It is eerie that two compatible young people say they will not marry because a four-year-old child forbids them to do it. It scares me still more when I begin to understand the mechanism at work."
"What is the mechanism, Minden?"
"The mutational inhibitions. It's quite a tangled affair. Do you remember the screaming monkeys of boondocks Rhodesia twenty years ago?"
"Vaguely. Bothersome little destructive monkeys that had to be hunted down and killed—hunted down by a sort of religious crusade, as I remember it. Yes, a mutation, I suppose. A sudden wildness appearing in a species. What is the connection?"
"Dismas, they were the first, the initial probe that failed. Others are on the way, and one of them will not fail. The story is that the religious crusaders said that no human child could be born while the howling monkeys flourished, for the monkeys themselves were human children. Well, they were. Well, no, they weren't children. And they weren't human. But, in a way, they had been both. Or at least—"
"No species can count itself secure that has not endured for ten million years," said Dr. Minden. "We still hear the old saying that evolution is irreversible. Hogwash! I have myself studied seven species of hogs washed away before one endured. The human race is so new that it has no stability. The majority of species do not survive, and we have lived only one tenth of the span that would tilt the odds in our favor. Even the species that finally survive will commonly revert several times before acquiring stability. We could revert at any time."
"Revert to what?"
"To what we were, to what we still are basically, little three-foot-high, big-headed, howling monkeys, without tools, and with only a fifth of our present life span."
"Did I say something wrong?" Ginny asked. "The last thing I ever say, and it should be wrong? Dr. Minden, you know about things like that. What are you creatures, anyhow?"
"People, Ginny," Dr. Minden said miserably.
"Funny I never saw any of you before. I sure don't intend to get involved with people."
Raucous howling! Hound-dog hooting! Hissing of badgers, and the clattering giggle of geese! Shag-tooth shouting, and the roaring of baby bulls!
And a screaming monkey leaped and tumbled up the rocks like crazy water.
