Yes a minor can drink alcohol legally with their parents in Missouri.
Although you still must be 21 years old or older to possess or purchase alcohol on your own you can still drink alcohol with your parents on your property in Missouri.
Missouri is the only state as far as I know that allows parents to serve their own minor children with alcohol.
You cannot serve other peoples children or minors with alcohol but you can serve and allow your own children or minors to drink alcohol.
If the minor is on their own property and their parents are present with them then they can legally drink alcohol.
I think it's a good thing as you can teach your children to drink alcohol responsibly.
As minors can still get a hold of alcohol no matter what and when they do it can be dangerous.
But allowing them to have some alcohol at home in moderation is a safer way for them to experience alcohol.
Alcohol in moderation can also prevent kidney stones and kidney disease.