NSPCC
The NSPCC provide lots of information and support to help keep children safe online, including parent guides and resources specifically for people with SEND
https://www.nspcc.org.uk/keeping-children-safe/online-safety/
Childnet
Advice and support for parents about how to keep your child safe online, including help with how to speak to your child about their online activity, gaming, online bullying and online wellbeing.
Internet Matters
Internet Matters provides support for families to help keep their children safe online.
https://www.internetmatters.org/
Particularly useful is their family digital toolkit tool - answer a few questions about online access and use in your home and get a personalised guide to improving online safety in your home: https://www.internetmatters.org/digital-family-toolkit/
They offer advice about important parental controls on a range of apps and devices: https://www.internetmatters.org/setupsafe/
They also have parent guides to popular apps and games your children may be using: https://www.internetmatters.org/resources/apps-guide/