Where the previous iOS keyboard app was iPad-only, DyslexiaKey works on the iPhone. From the store listing:
DyslexiaKey increases readability for those with reading disabilities.
Apple Distinguished Educator, Matt Thomas, has made a custom keyboard for iOS8, using OpenDyslexic. Below is the description:
Lowercase Keyboard is a custom keyboard that uses both lowercase and uppercase letters, as well as a simplified set of numbers and special characters. The keyboard has been designed with elementary students in mind.