Transcribed in great haste by his mother.
It reads: Jack’s pet loves slime. He likes to eat dirty stuff. He likes to eat dangerous hearts only if he is a grown up. He doesn’t like holes in him. This (arrow pointing to drawing of a box) is to keep him and you can see his tiny hole mouth. Do not make him die.
He gave this to his sisters. I haven’t heard a word about his invisible pet since, so I’m worried they didn’t follow the instructions.