APA is mostly concerned with the author as the creator of a work. However, when it comes to children’s picture books the illustrator often an integral part of the story telling process.
Follow the same format you would for writing an authored book reference. To acknowledge the illustrator, write the illustrator’s name as you would for an editor (initial, last name), follow with a comma and the abbreviation Illus.
Rusch, E. (2019). Mario and the hole in the sky: How a chemist saved our planet (T. Martínez, Illus.). Charlesbridge.
See APA Style post Children’s Book or Other Illustrated Book References for more on illustrated books.