Are there pictures of this?  I've seen pictures of boys wearing shorts pre Vatican II ... but never at Mass.

While it's unlikely to be an occasion of sin to anyone, it's still grossly disrespectful for them to dress that way at Mass.  Perhaps at a school "Field Day" or something, but at Mass?

SSPX is declining rapidly; it's difficult to resist the Novus Ordo contagion once you've put yourself in contact with it.

Are there pictures of this?  I've seen pictures of boys wearing shorts pre Vatican II ... but never at Mass.

While it's unlikely to be an occasion of sin to anyone, it's still grossly disrespectful for them to dress that way at Mass.  Perhaps at a school "Field Day" or something, but at Mass?

SSPX is declining rapidly; it's difficult to resist the Novus Ordo contagion once you've put yourself in contact with it.

You think the boys can wear shorts at a field day??

I don't know how to post photos on here, but I have a photo from May 1932 of my father's First Holy Communion class, Saint Nicholas R.C. Church, Jamaica, New York.  There are about 120 children flanked by priests and nuns.  The boys from about age 10 on down are all dressed alike in white shorts, white knee socks, white shoes with white shirt, jacket, ties.  The few older looking boys are wearing the same outfits with long pants.  The girls are all in identical long white dresses and veils.  Apparently, the sisters supplied the clothing which was either purchased or rented.  Many rented because it was during the Depression and families could not afford to purchase new clothes.  I can assure you everyone looks quite modest and dignified, shorts and all.  

I hope you're talking about more than just this, though, OP --
http://www.sspxflorida.com/en/news-events/news/classes-have-resumed-saint-thomas-more-academy-50431

The ones I saw were the older boys in the back of the picture, and they had shorts at mid-thigh above the knees.
Besides, shorts are not permitted at mass, it says it right in the bulletin and the entrance notice.

I don't know how to post photos on here, but I have a photo from May 1932 of my father's First Holy Communion class, Saint Nicholas R.C. Church, Jamaica, New York.  There are about 120 children flanked by priests and nuns.  The boys from about age 10 on down are all dressed alike in white shorts, white knee socks, white shoes with white shirt, jacket, ties.  The few older looking boys are wearing the same outfits with long pants.  The girls are all in identical long white dresses and veils.  Apparently, the sisters supplied the clothing which was either purchased or rented.  Many rented because it was during the Depression and families could not afford to purchase new clothes.  I can assure you everyone looks quite modest and dignified, shorts and all.  

The clergy in America has been infected with Americanism since the beginning, so it is certainly no badge of orthodoxy to find a pic of school kids in shorts when the clergy wouldn’t even wear cassocks! 

I don't know how to post photos on here, but I have a photo from May 1932 of my father's First Holy Communion class, Saint Nicholas R.C. Church, Jamaica, New York.  There are about 120 children flanked by priests and nuns.  The boys from about age 10 on down are all dressed alike in white shorts, white knee socks, white shoes with white shirt, jacket, ties.  The few older looking boys are wearing the same outfits with long pants.  The girls are all in identical long white dresses and veils.  Apparently, the sisters supplied the clothing which was either purchased or rented.  Many rented because it was during the Depression and families could not afford to purchase new clothes.  I can assure you everyone looks quite modest and dignified, shorts and all.  

From your post above, it would seem that it was acceptable among Catholics in 1932 that young boys age 10 and under could wear shorts, even for First Holy Communion. Which makes sense. It would seem that it might not have been appropriate for boys/men over that age to wear shorts. 
Trads tend to get a bit to pharisaical about such things, which makes some sense, in that we have to try to determine standards without having a proper church to guide us. 

I couldn’t find any sspx articles permitting children to wear shorts at mass.  Is this another unheralded change for the sspx?

I couldn’t find any SSPX articles permitting children to wear shorts anywhere.