if i was her husband i would've slapped her not that i encourage domestic abuse but if you as the mother slapped my child for simply asking a question.. i would do the same to you. bc that is a child and he was just asking bc it seems to have happened over and over so of course a child would ask. you could've apologised to the kid and say that its out of your control and we can celebrate his birthday after the brother recover. SIMPLE AS FUCK BRO. kid might still feel upset but he can also learn patience in a better context.