Pls lock in the Bathroom...Does it Work With All??

That only works if it is a place the child wouldn't want to be in. My kids would just start playing in the bathroom, lol. And I don't think it's a good idea to make the bathroom or cupboard a scary place. In order for it to work, the child would have to be locked in there to experience this as scary. Just saying, I'll lock you in the bathroom or cupboard won't work unless the child already thinks of it as a scary place he/she doesn't want to be alone in. Furthermore, you will have to follow through and really lock him/her inside the bathroom/cupboard, if he/she misbehaves after the warning or you render this disciplining measure ineffective. My guess is that your friends have locked their child in the bathroom, maybe even with the light off, repeatedly, or he wouldn't jump to attention as quickly as you indicate. That's traumatizing for any young child but especially at that young age. Yes, it is effective once the child is deathly afraid of the bathroom/cupboard/closet/basement, but the psychological damage is terrible. They have scarred their child for life. Do you want that for yours?

That seems strange to me. Plus I would not want a 3 year old locked in the bathroom where he could get into many dangerous things in the cabinets. In a few years he will probably be locking her in the bathroom.