How do I add new pronunciations to apple dictation?
I don't think that there is anything that is adjustable for Apple's dictation.  I assume that you could use Text Replacements, but I don't think they are interactively called when dictating.
You might have more control if you used a third-party dictation software like Dragon Naturally Speaking.
[Update]
My kids are dyslexic as well. I have used the text to speak to help them with their papers. I tell them to  just their their thoughts out onto paper before going back and fixing any little issues that have come up from the dictation. It's also a way for them to proofread their work to make sure they make sense.