I've seen the Back To The Future movies over a hundred times each, no word of a lie, I even watched the 2nd and 3rd last week in the evening with the missus, and I never ever saw this...
As some astute fan has discovered, Verne (one of the Docs two kids) is either a freaky little kid who doesn't know what he's signalling here (unlikely) or a perverse dwarf actor who seriously wants a bit of Elisabeth Shue... or Michael J Fox... I mean look as his smirk when he's doing it! Either way this creeped me out and made me laugh at the same time. Weird.