In the northern Europe, kids, also not grown in nudist families and not in nudist places, are usually used to go naked in the locker rooms, also with grownups around, naked as well. They get showers and saunas naked, never seen someone with swimsuit on.
They are perfectly comfortable with it because it's normal; sounds stupid to take a shower with swimsuit on, the only reason would be the modesty, but, evidently, it's so natural and normal for them, since they are toddlers, that nobody cares.