And if your preschool has a policy in place forbidding casual dress, well that is a signal to find another preschool.
The Firebug headed off to school this morning (first grade) in sweats, which may be a first; he almost always wears jeans. But it’s what he felt like wearing, and he doesn’t have PE today, so my wife and I couldn’t see any reason to criticize his choice. Didn’t know we’d stumbled into a faux-pas. 
Not picking on you in particular, since there were a bunch of posts saying essentially the same thing. What I don’t get, for all of these, is the ‘Is this the hill you wish to die on?’ thought.
Suppose the OP just ignores her text. What’s the downside for him? The downside seems to be with the ex, who spends her life getting mad over really small and silly stuff.
It’s not like she can take him to court for violating the terms of the divorce settlement, which I’m sure didn’t say anything about sweatpants on children.
Seriously, what am I missing here?
Slightly off topic I guess
But one thing I have learned recently is
"is this the hill you wish to die on?’ only works when you are dealing with rational, sensible people.
There are some people that if you give in on the small things they will push even harder to get their own way all the time.
The kind that ‘if you give an inch they take a mile’.
It often starts with little petty things and then escalates into more important things.
Sometimes it’s better to stand your ground than to try to reclaim it later.
I don’t see what the big deal is about a little kid wearing sweat pants (tailored ones at that) to school. Yes, appearances matter, but as long as your kid is reasonably neat looking at the start of the day, that should constitute “keeping up appearances.”
And by “neat,” I mean:
- clothes (sweatpants or not) are reasonably tidy - shirt is either tucked in or not, clothing is not stained or smelly, shoes are tied, the clothes actually fit and cover those portions of the body they’re supposed to cover
- hands and face aren’t covered in food and nose is wiped; hopefully teeth are brushed (though my kids sometimes escape out the door before accomplishing this)
- hair has had some attention (some hair just doesn’t lend itself to tidy, but if you’re a girl with long hair, it should probably at least be in a ponytail or headband or brushed; if short hair (boy or girl) try not having it stand up in tufts)
- Kid isn’t smelly
My children are prone to wearing the same clothes to school day after day. My rule is that they get to wear the same outfit twice in a row (underwear is one use only and if you take it off it has to be replaced), then they have to wear something else until their favorite clothing has been washed. I sure the heck hope no other parents notice that my son sometimes shows up to school in the same shorts and t-shirt twice in a row, much less the same running pants.