It has always baffled me how people get so offended when they incorrectly identify the sex of a baby.
Until puberty, the two sexes are quite similar and boys can easily look like girls and vice versa. The only main distinction that we have as the children grow is the way they style their hair and the way they dress because of our cultural norms.
With babies not having (much) hair to style and short of wearing pink or blue all day every day, it is quite conceivable to mix up boys and girls.
So parents, chill out: it is not offensive, nor a slight on your child’s appearance – just a biological fact that there is not much to distinguish them until puberty hits and their bodies start morphing into the adults that they will become. Just a simply-stated correction will suffice.