There was a thing 2-3 years ago around me where you'd go in Mcdonalds and they had several SSIDs in there called MCD0NALDS MCDWIFI MCDOONALDS MCRONALDS etc. If you connected to any of them, the sign in page would Goatse you.
I suspect it was a plug in ESP32 dongle or something hiding in the restaurant.
They likely would never had to explain themselve anyway. The fact the device is found doesn't mean the prankster would get caught. It is not like they will look for fingerprints and ADN for a small prank.
Unless the parent complains to TV news, who get B-roll of the McDonald's playground and Disney and other symbols of American childhood wholesomeness, and spin a narrative about some cyber-predator using computer hackery to reach out and traumatize children. A predator who apparently was physically at the location, possibly returning frequently to observe reactions to their prank, and who knows what all their intentions. Then they might become a priority to catch.
But my point wasn't that they'd get caught, but that those who were only (non-sociopath) pranksters would feel bad, when they started to realize that pranks can have serious unintended effects for themselves and others. Few teens would intentionally show goatse to a young child. The prankster just didn't think this one through, past "wouldn't it be funny if someone saw goatse due to a clever hack of mine", past the misdirected teen impulses to stir things up.
I suspect it was a plug in ESP32 dongle or something hiding in the restaurant.