I can't see why not.I would guess that those little girls can join that parade under some other umbrella than the GG. Maybe there's a "Santa's Little Elves in the Xmas Parade" group or something like that?
The entire Girl Guide troop could submit a float under another banner.
Maybe I'm wrong, but if Girl Guides Canada is explicitly threatening to kick girls out if they participate in a parade in any form, then that is something worth getting outraged about.