Mammele – if the child hasn’t seen it coming, then that means they still have. If he has seen it coming (e.g., there are no Shabbos seudos at home because they eat at kiddushim, there is no heat in the house, rain and mice are running through the home and it is in forclosure due to non-payment of property taxes), then it is expected (or he could get a Job and be responsible for himself, Chas V’Shalom). Besides, you should pay the electric bill and taxes before buying the Kallah a Rock (AYLOR). The slap is only if it is a shock, and then yes, Dei Machsaro applies (as he had more).
Your use of an extreme does not help your case.