in terms of pretend play with kids, honestly I just think there is no issue what kids wear. And most people know with very yong kids like toddlers, therer is really no issue either.
As you say, with other settings, we can tell kids it's not the social custom. That's what it comes down to, it's a social custom, but not that important of a social custom, because really it's not a big deal, inherently, what people wear.
But if we accept that it isn't a big deal, there isn't really much reason to go out of our way to flout the custom. No one has a deep internal need to wear a dress. It's not a big deal if social custom doesn't usually have men in dresses.
What that means is that the men who go out of their way to do it in any case are often doing it very deliberately for a reason. It just might be that they are into being very fashion forward. Maybe they want to be noticed. Maybe they want to push boundaries.
The idea that if only we didn't have customs like this, people wouldn't be needing/wanting to push boundaries, I think is a little naive. For one thing, there will always be culturally define social customs and boundaries. I think that even the more arbitrary ones work to give shape to people's lives in a way that is often useful. For another, there will always be people who want to push boundaries, and they will find them one way or another. It's not really about the dress or pants or whatever.