In English, "guardate tutti le ragazze" would become "all of you watches the girls," which is equivocal to "guardate le ragazze" with the added emphasis on the "voi." You haven't actually said anything different.
Oulenz has it right. You are limiting "you all," which describes totality of the directly addressed, to some kind of universal statement that goes out ad infinitum. "You all" is a relative terminology that varies based on the group, while include all within the group being addressed.
The problem is that the simple "you" is an ambiguous term, leading to confusion because of its inability to be accurate. You intended to say to the two people that you want them both to do something, saying: "you will watch the girls." From the listener's point of view, to whom were you talking? There are three options: person A, person B, or both. In the present moment, there would be other clues, such as: hand signals, eye contact, and inflection of the voice. These might help to clarify the situation at the time. However, these can be misconstrued or misunderstood, if they are received at all. If it's written or spoke over a phone, forget about it.
You don't have to like English grammar. However, "you all" is the correct grammatical reference for the second person plural. You don't have to change your English parlance to include "you all," however to learn Italian it would serve you well to have some grammatical clarity.
When I just did this exercise, it was in the form where all 6 forms of the verb conjugation were presented. But the "Io" form was mis-spelled, I think. It had "gurado". Wouldn't it be "Io guardo"? I know it's not the right form for this sentence, but I was looking at all the forms and it made me wonder.
I wanted to report it, but the only option was that the Italian sentence was wrong, and of course that is not the case. This is a different (possible) typo.