Shouldn't it read, Boy's Scout, Girl's Guide?
Boy Scouts (and Girl Guides) is a collective noun, although you can think of "Boy" as an adjective (it describes the type of "Scouts").
In my post above the lower case "scouts" and "guides" suggests that these are now to be read as verbs - indicating an action (e.g. "scouting") rather than a noun (i.e. a "Scout"). So the "Boy" and "Girl" now are the only nouns in the phrase.
In your post the apostrophe indicates a possesive - e.g. the "Scout" belongs to the "Boy" and the "Guide" belongs to the "Girl".
So the three phrases all have totally different meanings.
Edit: Of course the purists will notice that I have erroneously used the term "collective noun" when in fact it is a "compound noun".
The collective noun for boy scouts would be a "troop" or "patrol" and for girl guides it would be a "giggle".