You are conflating two separate and distinct legal issues. Children 5 and over may consume alcohol at home or on other private premises. Children are not permitted to drink alcohol in public places, such as parks or on the streets and the Confiscation of Alcohol (Young Persons) Act 1997 empowers the police to confiscate alcohol and arrest an adult supplying children with alcohol in public places. It is this law the gov.uk website is referring to.

An airport (and an aircraft) is not considered a public place for the purposes of that Act, they are considered private premises.