Has anyone been prosecuted because a minor drank alcohol for religious purposes, such as sacramental wine used for the Eucharist/Communion in some Christian denominations? This could be either the minor being prosecuted for illegally consuming alcohol, or another person being prosecuted for giving alcohol to the minor.
No. Not to the best of my knowledge. To quote Wikipedia on the subject found here:
In the United States, the minimum legal age to purchase alcoholic beverages is 21 years of age; the two exceptions are Puerto Rico and the Virgin Islands where the age is 18. The legal drinking age varies by state, and many states have no age requirements for supervised drinking with one's parents or legal guardians.
Presumably an individual's legal guardian was present during a service involving sacramental wine. For private religions schools, the solution was to not allow sacramental wine to be consumed during religious services conducted on student's behalf (Having attended Catholic School through elementary, middle, and high school, I can assure you that we did go to mass during school hours numerous times and in those occasions, wine was consecrated as part of the ritual but never offered to the congregation. Only the host. In fact, as part of the First Communion education, teachers made it clear to parents that the parents should allow the children to sample wine prior to the First Communion so as not to encounter a scenario where the child would discover they did not like the taste and made a face or spat out sacred wine in disgust. They even gave parents the name of the brand of wine they used for the purposes of making sure we were sampling the right type. I remember I did not like the taste and haven't had wine since.). It should be pointed out that this existed at a time when anti-Catholic sentiment in the United States was quite high (The U.S. is one of the few nations with a Protestant Christian majority) and anti-alcohol sentiments may have been rooted in anti-Catholic and anti-Immigrant sentiments of the time.
Even under the Volstead Act, the bill that allowed for the enforcement of prohibition under the 18th Amendment, a special exception was allowed for religious consumption of alcohol, and a religious official could receive a permit for the procurement of alcohol for such purposes:
Pastors, priests, ministers, rabbis and others who practices religious actions could acquire a permit to provide alcohol for sacramental purposes only.
What's more, any enforcement of the law against a religious organization would be a violation of the free exercise clause of the First Amendment which prohibits any law that violates the practice of religious tenants.
While sacramental alcohol has, to my knowledge, not been involved in significant U.S. Case Law to the best of my knowledge, that does not mean comparable case law does not exist. For example several traditional Native American religions use peyote (a Schedule I controlled substance), did result in several state laws and U.S. Federal Law enacting exemptions for its religious use. In 1990, the Supreme Court held in Employment Division v. Smith that laws prohibiting the use of peyote that do not specifically exempt religious use nevertheless do not violate the Free Exercise Clause of the First Amendment.
Considering that alcohol is a less restricted substance in U.S. Law, it would hold that if the enforcement of any drinking age law without exemption for religious use would be unconstitutional. In the case of Employment Division v. Smith, many states rewrote their controlled substance laws to include such exceptions prior to the decision. At the federal level, this exemption was already codified law, as part of the "American Indian Religious Freedom Act" which was passed in 1978.
It is not illegal for a 5-year-old to drink alcohol on private premises.
The registered charity Drinkaware says
In England, Scotland and Wales, it’s not illegal for someone between the ages of five and 17 to drink alcohol at home or on other private premises.
However a church is a public place and there may be exemptions to the law for religious purposes. Wikipedia states in Religious exemption
A religious exemption is a legal privilege that exempts members of a certain religion from a law, regulation, or requirement.
. . .
Laws governing alcohol may sometimes grant exceptions for practices such as Eucharist .
The Eucharist is also known as Holy Communion.
There is an annotation 'citation needed' and I haven't been able to track down the exemption, but bear in mind that communion is one sip of wine, at the very most. The Church of England says
From the age of five, parents may allow a child to consume an alcoholic drink at home, but they may not buy their child an alcoholic drink in a public house or restaurant until the age of 16. The administration of communion wine at a service in church or school does not fall into either of these categories, but it is reasonable to suppose that a parent may give permission for any child over the age of five to take a sip from the chalice. The amount of alcohol consumed is tiny (< 0.04 units; even less if a generous quantity of water is added in the preparation of the chalice).
In UK, where the police don't have enough resources to deal with low level fraud and burglary, the chances of anyone being arrested for serving communion wine to a minor are vanishingly small. If they were, it would probably involve a much more serious abuse charge.