You think the boys can wear shorts at a field day??
What on earth is wrong with that? Shorts have always been a common attire for young boys, especially when doing outside activities. Wearing shorts to Mass is an issue, but wearing shorts in general is not. Now, for older boys it can be a bit infantile(well, going by early to mid 20th century fashions at least, there's nothing intrinsically immature about them) unless they're playing sports, but for kids absolutely not. That's traditional attire for them.