Being able to touch type is an invaluable skill for children (from about the age of 8) and adults with dyslexia. Once touch typing becomes an unconscious skill the writer has more capacity to process thoughts and think about what they are writing. Touch typing removes the need to physically move a pen while thinking of how to spell a word, something that those with dyslexia can struggle with.
Touch typing aids spelling as well because fingers learn patterns for common words and so you don’t have to consciously think about how to spell, your fingers just do it for you!
There are both bought and free touch typing tutors for all ages. However with dyslexic children it is probably best to buy one specifically written for them, such as Englishtype (https://englishtype.com/) or Nessy Fingers (https://www.nessy.com/uk/product/nessy-fingers/). With older teenagers and adults Kaz Typing Tutor (https://kaz-type.com/) or Mavis Beacon Teaches Typing (http://www.mavisbeaconfree.com/) would be good. As with all skills practise makes perfect, but the rewards are worth the initial effort.