Bicycle Tutor

My grandson has a bike that has a wobbling crank, and this in turn makes the chain jump off. Do i just need to grease it through the hole in the bottom or do i need to take the whole thing apart? thanks

By wobbling, do you mean that it has side to side play (you can actually rock the crack arm left-right)? Or that the chainring is not vertical and moves in and out as it goes around?
In the first case, the bottom bracket (bearing assembly) probably needs adjusting or overhaul. Second case, could just be the arm needs to be tightened, or could be bent.