We all read things with our own bias and experiences, which influence our interpretations.
Most of us who are parents read the boy's comment as not rude at all, just brief and to-the-point, which is most likely based on how we text with our own kids.
But, if someone was used to receiving demanding texts, I can see how that person might assume that was how the text was intended. Not saying that's the case with the OP's kids, of course, but perhaps she deals with entitled people IRL, coloring her perception of the boy's words.