For the first film, the people behind the scenes of the movie decided to use an older actor for Mary Marvel, but opted to use Currey in a dual role.

Source: https://comicbook.com/movies/news/shazam-fury-of-the-gods-star-grace-caroline-currey-says-adult-counterparts-acted-younger-than-kids/

While Shazam himself is still firmly at the heart of the film, his foster family all get their own showcases -- especially Mary Marvel, as played in both her civilian form and superhero persona by Grace Caroline Currey.

Source: https://www.cbr.com/mary-marvel-best-hero-spinoff-dc-movies-shazam-fury-of-the-gods/