I made a mud pie and told my kid brother it tasted like chocolate so he'd try it
That's a mean prank. Why would you tell your brother that?
I was kidding. Eating some mud is good for your immune system
I'm not sure you're correct about his immune system. It's not okay to trick someone into eating something they don't want to. Your kid brother could get very sick.
I don't think there's a risk anyway.
Your brother could accidentally eat a stone and choke. Plus, it's just wrong to do because he wouldn't trust you after doing this.