Well maybe some people do get upset when people misgender (or mis-sex, or however you want to say it) their child, but I'm sure the majority of us reasonably minded parents brush it off.
If my little girl isn't wearing pink most people assume she is a boy, but I don't get upset about it. It's just one of those things.
Yet I'm going to a ticketed event soon where the organisers have announced (long after the tickets were all sold) that we must refer to every single one of the 2k other people as 'they', until we know for definite, the gender of the person. Why does everyone have to make such a big deal out of these things for adults? It's the first time I'm going somewhere social without kids in two years and I feel like I'm stepping into a very childish world compared to the parent world.