Exactly how does one know when to you vous and when to use Ils

Question

I was never aware "JACK and JILL' should take the pronoun Ils.   It was my belief ILS was reserved for all boys only.  
So now I see vous addressing the individuals IN the group.   Sort of like ILS is they and VOUS is similar to America's deep South as y'all, or the northeast as in you guys. 
So within the poem/song Jack and Jill went up the hill one wouldn't use the pronoun VOUS as it would change the meaning.   Is that correct?
Not Y'ALL or You guys or VOUS     went up the hill...  Jack and Jill are not the audience.
THEY or ILS went up the hill to fetch...  The audience is being addressed.  Not Jack and Jill.
Dear professor, is that about right?

Accepted Answer

Vous is the 2nd person plural, you use it when you are addressing more than one person, or when you are talking to a single person and address her formally.
Anne et Marie, vous allez au cinéma?Jean et Pierre, je vous ai vus hier.Madame, qu'est-ce que vous faites?
Ils is used as a pronoun for the 3rd person plural ("they" in English). If you are speaking about an all female group it is elles.
Anne et Marie, elles sont allées au cinéma.Jean at Marie, ils m'ont vu hier.
I hope that helps.