I am just not sure if its taboo at a 5 year old party.
I don't think it's taboo
but, it certainly depends on the audience. I wouldn't at my son't birthday with a handful of the two-bob snob parents. I've heard them gossiping about other parents about a wide range of petty things from not being plastered head-to-toe in sunscreen, to someone catching nits to just generally criticising anything that moves (for the sake of it).
I personally wouldn't bat an eye lid at having a civilised glass of wine or beer. I wouldn't be impressed if the hosts were throwing back a dozen Wild Turkey cans
and playing drinking games!