Ben Levi –
In regards your kiddush vs. purim argument every state has different rules about the amount of alcohol a parent can give their child and in what setting. for example – in New York a person under 21 may consume alcohol in their own home, with their parents consent. They can also consume in a licensed restaurant, with a meal, if the parent actually pours the drink. (Mere ‘consent’ is not enough)
It is also illegal, in NY, for a minor to have a BAC above 0.02% at any time. So while it’s legal for a parent to serve their child wine with a meal, for example, they must restrict the amount to ensure they remain under the 0.02% limit.
Most states have very similiar rules in this regard. So it makes sense a person under 21 can make kiddush at home but not get drunk on Purim.