At this time, your child is likely to be spending a lot of time online at home. Try to set up a space for your child to work downstairs in a location where you are around. Not only will this enable you to work with them and help keep them motivated, it will enable you to see what they are doing on their computer, iPad, phone and games console. You may want to contact your Internet Service Provider (ISP) and implement appropriate filtering settings if they are not applied already. Mobile phone providers can also provide filtering services on the 3/4/5G networks now, so worth contacting them as well. Don't forget, many mobile devices can be used to 'hotspot' a WiFi connection (this allows any WiFi device to use the mobile data on the phone), so just switching off your router or filtering a single PC will not necessarily solve the problem.
If you need further guidance on keeping you child safe online:
Child Exploitation and Online Protection
If you are concerned about online abuse or the way that someone has been contacting you online, please make a report to CEOP.