A good way to supervise children’s internet access and set boundaries about what they can and cannot do online is to create an agreement with them.
If a child breaks the rules, you could restrict internet access for an agreed period of time.
Ensure you use the privacy settings, parental controls and built in internet safety features provided by the major internet service providers.
The UK Safer Internet Centre has guides for parental controls - https://www.saferinternet.org.uk/advice-centre/parents-and-carers/safety-tools-social-networks-and-other-online-services
For parents and carers experiencing any internet safety issues with their children, The Parent Zone (parentzone.org.uk) provides a national helpline service at - firstname.lastname@example.org and via their website.