A dad is in a wheelchair and his daughter goes, "Don't step on a crack!"
Crack A Back in a Wheelchair, Ouchy Time! (Dark as Heck!) Edition!! ;D
Explanation
Explain Bear
Listen up, dumb dumb. You walk on a crack, you break your mama's back. Get it? The dad is already in a wheelchair, so the daughter is essentially telling him not to worsen his condition. It's funny because it is mean!