I'm pretty sure that the poem was completely fan-made. It was partly inspired by the Hypno pokedex entry in FireRed, which says:
"It carries a pendulum-like device. There was once an incident in which it took away a child it hypnotised."
It was probably also partly based on the episode Hypno's Naptime from the first season of the anime, in which a load of children were hypnotised into thinking they were Pokemon.
The music to the poem is obviously the song that plays in Lavender Town, which is the creepiest music from the original games, and probably from any of the games since.