Wu is referring to The Enigma of Amigara Fault.

↓ Transcript
Selkie: THIS is MY BONE! There ares many bones likes it, but THIS bone is MINES.

Sandy: That’s co cool!

Wu: Ha ha hey, Sandy, that reminds me of the story Dad read us last night.

Sandy: Huh?

Wu: Yeah, that spooky comic?

Wu: “This is my hole! It was made for me!”


Wu: It’s about people being summoned into haunted torture caves.

Selkie: Gives to me this story.

Wu: Heh heh...

Enigma of Amigara Fault is technically not inappropriate, but I'd still give your kids their brown pants before reading.