@sgamer82:
A lot of the potential trauma is probably lessened by things like how kindergartner quirks don't seem especially powerful (Bakugo only made his hands spark), quirks in general but being all that powerful (it's more uncommon than otherwise to have a hero level quirk), and the fact a kid is most likely inherits one of his parents' quirks, so they can get an idea what to expect and parents can prepare for it.
So I imagine Mirio's mom or dad gave him an idea of what to expect when his quirk was about to come in. Even then, odds are he got introduced to his quirk the same way we did, his clothes spontaneously falling off. Kid probably wouldn't even care at that age.
There could still be issues if one or both parents aren't involved in a kid's life, or if the child develops a new or hybrid quirk. I imagine it was most traumatic for the first couple generations.
That doesn't mean it still can't inflict trauma. If anything it makes for more unique scenarios. Like his parents being overprotective of him, especially if he tells them he wants to be a hero. Having his clothes fall off in the middle of class regardless of age would get him humiliated (kids would definitely care at that age, you underestimate how ignorant and judgmental kids can be, especially if they keep ending up in your classes. Just look at the start of Chapter 1) and even thought to be creepy. Especially if he got accused of using his powers of being a pervert, cheating, stealing, etc. Although that gets into elementary/high school territory. And just because his parents may become permeable does not mean control can be learned verbally. It can be a situation that can only be acclimatrd to through a first-hand reflex, which can be very scary. This is how plenty of fictional heroes get the hang of abilities like flying or heat vision. Just because you have parents with similar powers doesn't mean it becomes a breeze to learn how work and deal with other people's perceptions.
After seeing Todoroki and Shinsou's backstory, I don't have any doubts about Mirio going through some sort of rough childhood. It's begging to happen with the whole "it's not about the Quirk, but how you use it" theme and how the kids think he is untouchable (lol).