There are two things under discussion here, I think. One what precautions do we need to take to be reasonably sure that our GC are not targeted by perverts?
Here I don't really think their clothing is the issue, although too skimpy garments probably do catch a pervert's eye. We, the grown-ups, need to be on the watch for adults hanging around playgrounds, or staring over our garden fence at children playing. Sadly I think we have to assume that any person who we do not know might have ulterior motives if he or she (yes, there are women among the sexual predators, usually more interested in little boys than little girls)is hanging around staring at children.
The other aspect of shorts under dresses or leggings is that little girls have always teased each other, and little boys teased little girls if the girls' pants could be seen when climbing trees. Most of us dealt with it by telling the others that they were rude for looking up under our skirts, but it's fine by me if little girls want to wear an extra layer under their skirts. I'm not so happy at the thought of parents demanding they do so, but it is after all up to parents to decide what clothes their children should or should not wear.
I distinctly remember wriggling around trying to get out of a wet swimsuit, dry myself and get into my clothes under a large beach towel. In the teeth of the kind of gale blowing on the average Scottish beach that was a difficult exercise, but modesty forbade "wee gurrils" in their nothingness on the beach in the 1950s, and I don't remember any "wee willies" on show either, so the rules must have applied to wee laddies as well.
After the permissive society of our young days, the pendulum was bound to swing back, again. Grandmothers who had grown up in the kind of clothes Napoleon's Josephine wore must have felt the same at the start of the Victorian era - there is nothing new under the sun,