Grumkins as another species akin to the children of the forest, giants and humans. Asha says as much.
"This southron king seemed to be one of those men to whom women are another race, as strange and unfathomable as giants and grumkins and the children of the forest." - The King's Prize ADWD
Tyrion is dismissive of their existence.
"...on the Wall, watching for grumkins and snarks and all the other monsters your wet nurse warned you about. The good part is there are no grumkins or snarks, so it's scarcely dangerous work." - Tyrion II AGOT
Arya considers the dangers of grumkins granting wishes.
" In Old Nan's stories about men who were given magic wishes by a grumkin, you had to be especially careful with the third wish, because it was the last." - Arya IX ACOK
Sansa thinks about them as well.
" In Old Nan's stories the grumkins crafted magic things that could make a wish come true." - Sansa V ACOK
Dany receives the blue drink from a "dwarf" outside the House of the Undying.
"...the smallest dwarf Dany had ever seen was waiting on the threshold. He stood no higher than her knee, his faced pinched and pointed, snoutish, but he was dressed in delicate livery of purple and blue, and his tiny pink hands held a silver tray. Upon it rested a slender crystal glass filled with a thick blue liquid: shade of the evening, the wine of warlocks." - Daenerys IV ACOK
The description of this creature is quite different from all the other dwarves described so far.
"He was a dwarf, half his brother's height, struggling to keep pace on stunted legs. His head was too large for his body, with a brute's squashed-in face beneath a swollen shelf of brow." - Jon I AGOT
A squashed-in face being the opposite of pinched and pointed.
Inside the House of the Undying, Daenerys sees a vision wherein four of these "dwarves" ravage a beautiful woman.
"In one room, a beautiful woman sprawled naked on the floor while four little men crawled over her. They had rattish pointed faces and tiny pink hands, like the servitor who had brought her the glass of shade. One was pumping between her thighs. Another savaged her breasts, worrying at the nipples with his wet red mouth, tearing and chewing." - Daenerys IV ACOK
I'm reminded of the way Mad Aerys savaged Rhaella.
"They said the queen looked as if some beast had savaged her, clawing at her thighs and chewing on her breasts." - Jaime II AFFC
The Mad King seems to have gone mad after his captivity at Duskendale. Did a grumkin grant his wish to be rescued? A wish that led to the end of his family dynasty? (Was Aerys replaced by four grumkins stacked up wearing a cloak? j/k sort of).
The visions in the House of the Undying are, in a sense , offers of wish fulfillment. Many of the visions have to do with her lost family members and even a return to the house with the red door where Darry tells her she's home and safe, possibly the thing she wants the most.
"All that Daenerys wanted back was the big house with the red door, the lemon tree outside her window, the childhood she had never known." - Daenerys I AGOT
She refuses all of these visions.
Good for her.