Many states have a provision allowing minors to consume alcohol provided by parents/guardians, but good luck getting a bar to serve to a minor, even if the parent is there.

No bar owner wants to deal with nitpicking from law enforcement over whether or not the person with the minor was actually their parent or guardian, and serving someone that is obviously a minor is a big enough cultural taboo in the US that few if any business owners/managers are willing to do it.