I was never slapped at my Confirmation and neither was my mom and she is almost 60. I have no idea what the purpose of slapping a child would be. I would have serious issues if the Bishop slapped my child, religious purpose or not.
Vatican II happened before I was confirmed - but the idea of slapping somone at Confirmation was to remind them of the persecution they will have to suffer for Christ. And it wasn't a big slap where one ends up with a red mark or anything - it was more a pat on the cheek. And it's a custom/tradition/discpline so it wasn't done with every Catholic Confirmation.