I hold Dolly to rasom if Sarah does things she knows she isn't allowed to do, like pushing or damages something that she is not supposed to touch. She has to apologise and give a cuddle before Dolly comes back.

I don't think you are being too harsh. Think about it this way, You are not hurting him. You are not doing anything that can injure him. You have told him how to get it back and he is refusing. What he did could have injured him (Water is a conductor and he could easily have gotten an electric shock.) I think you are doing the right thing hun. He needs to learn that doing dangerous things has huge consequences.