It is to emphasise that it is the plural you and not singular. Without the "all" it is ambiguous to say "you", although "you" is still commonly used without "all" in standard English. Perhaps the phrase "Are all of you giving my cat milk?" would be clearer.
Perhaps that is true in American English, y'all, but definitely wrong in British English.
I think clearest would be if they just put in an indicator for plural as follows:
"Are you [plural] giving my cat milk"
- meiner Katze- used for female nouns
- meinem Kater - for male nouns
- meinen Katzen - for plural
and i add to BirgitAsh, the case you use here for "my cat" is called dative. You can have a look there for the 4 German, grammatical cases: https://deutsch.lingolia.com/en/grammar/declension