Trust me, at the Grotto they count. Honestly, I won’t send my girls to mass without sleeves and my parish is known as Grotto North.
But we are not quite as, serious, to put it nicely. Our woman wear pants sometimes.
But as far as dress goes, I would never, ever send my girl at any age to church without sleeves. If she wore a pinafore, she had a little blouse or turtleneck under it. No braids, and a bow in the hair. Even my Rebellious Teenage Son has always had to wear a collared shirt and no blue jeans for Mass.
But I don't think it's a modesty issue when they're little -- more of a 'respect for God's house' issue. Don't go to church looking like you're going to hang out and shoot pool . . . .
. . . the little flower girls don't look immodestly dressed, even though their arms aren't covered. Maybe the long skirts make up for the lack of sleeve? (and from the back it doesn't look like the gowns are low-cut, even assuming there was something to hide).