One of the most important things we teach is to read. Children can discover new worlds, and teachers and parents love to see children learning from reading.
There are many iPad apps to support the teaching of reading; some for children with little to no reading experience, while others for slightly older children who want to advance further.
Here are just a few suggestions of apps which support the learning of vocabulary and sight words.
Bitsboard free and has loads of word games
Starfall, Learn to Read contains mini-books free 
Sight words Eggy Words free for first 100 words
Sight Words list 0.99p