The following apps would enable you to help your child learn and/or revise their letter sounds.
Jolly Phonics (Android) Here (currently FREE)
Jolly Phonics (iPhone/iPad) Here (currently FREE)
Hairy Phonics (Android) Here (currently FREE)
Hairy Phonics (iPhone/iPad) Here (currently FREE)
To help your child to learn the first 100 high frequency Sight words. The following app from Nessy would be helpful moving on to Hairy Words 2.
Hairy Words 1 (Android) Here (currently €3.99)
Hairy Words 1 (iPhone/iPad) Here (currently FREE)