Simply put: Children learn from play. They learn through the actual "work" of hands-on investigations with their environment.
Want them to learn the alphabet, be great readers, etc? Expose them to written words everyday....read to them, point out words on everyday items, write in front of them (this can be the writing you already do, such as your grocery list) and let them be part of the writing, point out individual letters (starting with the letters of their name is great since toddlers are egocentric!), sing, rhyme, etc.
Exposing them to the meaningful ways we use letters does more for them than alphabet flashcards. Will your two-year-old (or even three-year-old!) be able to tell you what every letter in the alphabet is if you quiz him or her? Absolutely not! However, they will have a better foundation for reading through the meaningful, hands-on, real-world experiences AND have more natural motivation to learn. Think about it: are you more geared up to learn about something when it has meaning for you or when someone else wants you to learn so they can quiz you?
Want them to know their numbers, be math whizzes, etc? Give your child many opportunities to count in a meaningful manner. Think about the ways you use simple math in your everyday life and see how you can involve your child. Cooking is one routine way to add math. "We need three cups. Let's count them out." Point out times when there is more, less, some, all, none, etc. during everyday play. Let them experiment with measurements by filling the sink with water and various sized cups, containers, etc.
There is much research on the importance of play. A great place to start is a book titled "Einstein Never Used Flashcards: How our Children Really Learn -- and Why They need to Play More and Memorize Less" by Kathy Hirsh-Pasek Ph.D., Roberta Michnick Golinkoff, Diane Eyer.
Stay tuned for more ways to play with your child and teach them in meaningful ways that will go deeper than any flashcard or "educational product" on the market!