I remember watching this movie between 2016 and 2019 and it looked new.

After a party in this city, all the adults start acting like little children, so the children investigate what is going on, in the end they discover the party’s drinks were contamined with a drug created by the government a long time ago in a scientific experiment in the city. The experiment ended because the test subject were dying.

Also the main villain is a woman who wears all black and is the daughter of one of the scientists who conducted the experiment, and before the end she participates in a boat chase in a river.

Looked like an european movie (british?)