As a minimum, ensure that your own home broadband has ‘family friendly’ filters enabled by default. All major broadband providers are required to have these filters in place.
UK Safer Internet Centre has resources and advice about internet safety specifically for adoptive parents at https://www.saferinternet.org.uk/advice-centre/foster-carers-and-adoptive-parents
Childline has some useful resources about online and mobile safety that are child friendly at https://www.childline.org.uk/info-advice/bullying-abuse-safety/online-mobile-safety/
You can download useful leaflets with advice for keeping children and young people safe online as pdfs below.