It is NEVER legal for any adult to provide alcohol to someone else's child, even with the permission of that child's parent. Parents who host, lose the most.
The providing host can be charged with Unlawfully Dealing with a Child in the First Degree, a misdemeanor carrying a punishment of up to 1 year in jail, up to 3 years of probation and up to a $1,000 fine. These are the Criminal Law sanctions ... Civil Liability may also exist. There are many counties and municipalities who have also adopted local "Social Host Laws" that further the liability of providing alcohol (and in some cases drugs) or a place to consume them to an underage person. No one wants to be the parent who is awakened in the night to be informed that his or her child has been injured or killed in an impaired driving crash.
Stay safe and keep your children safe. This is a time of year for joy and celebration, do no be the host of an underage drinking party - it's not worth it.