[Steve Van Doorn]

Since you like Billy & Mandy and I only watched it a few times. Why can’t the Grim Reaper kill those kids or other people? How does Mandy have him under his control?

[Starter]

"The Grim Adventures of Billy and Mandy is the story of two kids, the dimwitted Billy, and ever cynical Mandy, who meet Death himself (Grim). In an effort to save Mr. Snuggle's life (Billy's pet hamster), Mandy purposes a game with some high stakes. If the Grim Reaper wins, he can take Billy and Mr. Snuggles to the underworld, if the kids win, the Reaper is forced to be their best friend forever. After a grueling limbo contest, Billy and Mandy find themselves a new playmate for life."

[antiRepublicrat]

Since you like Billy & Mandy and I only watched it a few times. Why can’t the Grim Reaper kill those kids or other people? How does Mandy have him under his control?

They won a bet over Billy's dying hamster, limbo dancing, IIRC. Grim lost and has to be their friend forever.