There are many of them for all ages and some games can be used even for smaller children than it's mentioned in game's description. First of all, I recommend you to look through such resource as ABCya!, it's a big site with many teaching games, useful books and other tools for reading. And all games there are free, only in case of apps you'll need to pay for special subscription.
In case of certain games for such age I recommend you The Amazing Alphabet Match-Up , it's a cool virtual card game, in some ways similiar to the classic live ones. Alphabet Antics is another nice one, it's funny, simple and contains a story about cute moneky who needs some help with coconuts :) Zebrainy ABC Wonderlands https://itunes.apple.com/app/apple-store/id1165324239?mt=8 is a good one too, it belongs to adventure/quest genre, has a very interesting plot, high-quality graphics and cool teaching method. Also try Fish School HD, it's very good not only for learning alphabet but also numbers, colours, shapes etc. It's also a very fun too.
I can recommend you to check Numberlys, Sesame Street, Scout's ABC Garden and Duck's Alphabet. But all these games should be always combined with other live methods for the best development of your child.