This link takes you to lots of useful resources about how to make complaints to the NHS, including easy-read information.
https://www.england.nhs.uk/learning-disabilities/about/ask-listen-do/people-information/?fbclid=IwAR2XMmWrIpzlRQRMCAW7jJd1bnTcUX-FSLTNHp45AbDjAbQ0yaz-9DcQy0Y#top-tips-families