Not a class you sign up for so much as a club you belong to. The same group of kids meets most Saturdays with the same volunteer monitors, year after year from age 5 into their teens, and the year builds toward camps and hiking routes the leaders plan themselves rather than a package you buy.
Esplais are volunteer-run weekend clubs where kids play, make things and go on excursions with the same group all year, a long-standing Catalan tradition. Esplac is the secular federation behind around a hundred of them, 24 of those in Barcelona, organized into three city sectors that between them cover most districts. Kids take part from age 5 to 17, and Esplac's online finder maps every club so you can contact the one nearest you.