We can see why a bunch of educational titles aimed at young children would be a smart marketing ploy by Nintendo. The Mario's Early Years! series was meant to rival the Blaster Learning System titles of the early '90s, as they covered similar subjects and took the same approaches to gameplay. There were three Early Years games: Fun with Letters, Fun with Numbers, and Preschool Fun, and they're all as simple as you could imagine. They each got an MS-DOS release prior to each of their Super Nintendo debuts, with an anthology trilogy later released for PC.
Mario's Early Years! Fun with Letters was meant to improve your vocabulary and grammar. You chose Mario or Princess Toadstool, a question would pop up on the screen (like a fill-in-the-blank question), and you sent them to the appropriate answer. You had to complete sentences, choose alliterations, or find words that contained a specific consonant sound. Fun with Numbers played similarly, but with simple math questions. Preschool Fun was probably the most advanced of the three, since it had you matching shapes, counting, and matching animals to their appropriate noises. But here's a better alternative for your preschooler: just let them play the Super Mario series like a normal, sane parent, and let them learn about letters and numbers in school.