In this article on the Language in Interaction NEMO Kennislink themepage: A beginning reader learns to link sounds to letters. But how do you do this if your mother tongue does not have any sounds, as is the case with Dutch Sign Language? That is quite difficult; deaf children therefore run a greater risk of becoming illiterate. Good reading education can prevent this (Dutch):
https://www.nemokennislink.nl/publicaties/leren-lezen-als-je-doof-bent/