[pay day]

In a private home, an adult party (40 to 60 year olds) asking 16 year olds to mix alcoholic beverages and pour and serve wine and beer. Is this against the law in California?

[Fr_Chuck]

A minor is allowed to be given alcohol by their own parents, it is in poor taste and llegal to have them be bartenders at a party

[AK lawyer]

a minor is allowed to be given alcohol by their own parents, it is in poor taste and llegal to have them be bartenders at a party

Fr_Chuck, I believe you may be mistaken when you write that one may give alcohol to one's children:

California Business and Professional Code Section "25658. (a) Except as otherwise provided in subdivision (c), every
person who sells, furnishes, gives, or causes to be sold, furnished,
or given away any alcoholic beverage to any person under 21 years of
age is guilty of a misdemeanor.
(b) Except as provided in Section 25667, any person under 21 years
of age who purchases any alcoholic beverage, or any person under 21
years of age who consumes any alcoholic beverage in any on-sale
premises, is guilty of a misdemeanor.
(c) Any person who violates subdivision (a) by purchasing any
alcoholic beverage for, or furnishing, giving, or giving away any
alcoholic beverage to, a person under 21 years of age, and the person
under 21 years of age thereafter consumes the alcohol and thereby
proximately causes great bodily injury or death to himself, herself,
or any other person, is guilty of a misdemeanor.
..." CA Codes (bpc:25657-25667)

I don't see a parent-child exception.

The reason it may not be illegal to allow minors to serve alcohol in the situation posed by the OP is that it's not a regulated establishment.