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. By using a font with a heavy base line, alternating stick/tail lengths and larger than normal openings, DyslexiaKey assures all users that each character will have a distinct difference, making reading/writing easier.
Get it: https://appsto.re/us/EOdf5.i
Developer's Website: http://rapkey.co/