Old enough to know that it can cause issues but probably not old enough to know that can cause serious issues along with him and his siblings being taken into care.
Tell your son, "Mommys who punch their children also don't give their children Christmas gifts." Should resolve his lying problem fairly quickly, at least hopefully!
I knew a mom that, after more than enough warnings, cancelled Christmas in her house because her kids were being horrendous. Not just typical excitement either. I'm still a little shocked years after hearing about it.
I agree. Don't threaten with something unless you're actually prepared to use it and follow through. Sounds like the kids tried to call her bluff and the mom had to follow through. As sad as that is I bet it taught them a solid lesson and I bet they shaped up pretty quick after that. Good on her for following through.