Depending on what state you live in, it may actually be illegal to serve alcohol to minors, even in a private home, particularly if you are not the parent or guardian of the child. You may also be held legally responsible for any damage caused as a result of the alcohol, particularly if the child is under the age of 16.
It is illegal in all states for non family members. Here is a quote from the NIAAA: (emphasis mine)
Some States provide an exception when alcoholic beverages are furnished to a minor by a parent/guardian or spouse. Some States specify that the spouse must be of legal age, while others do not. APIS considers a State to have a spousal exception even if the age of the spouse is not specified.
In some of these States, the exception for family members applies only if the furnishing occurs in a specified location, e.g., all private locations, private residences only, or in the home of a parent or guardian only. No State has an exception for furnishing on private property by anyone other than a family member.