My 5 year old doesn't like grandpa and he shows it loud and clear. He sticks his tongue out at him when he walks by! Grandpa (my dad) slapped my son's face the other day for doing this. I'm mad at both of them. I've told my son he has to be polite. My father shouldn't be hitting my kid (I know it's the old school of thought but I don't want him to do it to my children). What can I do about this?