Reason might be that it weren't kids in the variant of the legend the painting is based on, but a kind of medieval travelling clerics on their way to Athens. Says so in the German Wikipedia.
Medieval paintings didn't care for realistic size comparisons, so might well have been misinterpreted as kids in later times.

[–] WoodScientist@lemmy.world 7 points 3 weeks ago (1 children)

Maybe St. Nick half-assed the sign of the cross. So when the traveling clerics were resurrected, they got resurrected in fun sized.

[–] gandalf_der_12te@discuss.tchncs.de 2 points 3 weeks ago (1 children)

why is it called "fun" sized?

[–] Sculptor9157@sh.itjust.works 2 points 3 weeks ago

Because "lesser portion" doesn't hit the same.