When kids make chestnut males it's pretty common that a hand gets injured by the pricker. Using this simple construction it is much saver even for small kids, as the arm has a good leverage to penetrate the shell while the other hand can securely hold the chestnut.
The bolt needs to be sharpened with sandpaper and screwed in a pre-drilled hole. The position is marked on the draft. A machine bolt (size M5), two washers and a self-locking nut finalize the construction.