For giving alcohol to children the law varies state to state. In Ohio for instance, the law states: "No person shall sell or furnish any low-alcohol beverage to, or buy any low-alcohol beverage for, an underage person, unless given by a physician in the regular line of his practice or given for established religious purposes, or unless the underage person is accompanied by a parent, spouse who is not an underage person, or legal guardian."
New York State is different. While parents can consent to underage drinking, it has to be in the privacy of their own home, and the law is purposely vague, just saying "Moderate amounts" which essentially means you never know the limit until someone prosecutes you for it. The beverage must also be served by the parent, and they may not serve anyone except who they are legal guardians for, so no going to a friends house and having his parents give you wine.