Current Government Guidance
People with symptoms of a respiratory infection including COVID-19 - Guidance for people with symptoms of a respiratory infection including COVID-19, or a positive test result for COVID-19. https://www.gov.uk/guidance/people-with-symptoms-of-a-respiratory-infection-including-covid-19#children-and-young-people-aged-18-years-and-under-who-have-symptoms-of-a-respiratory-infection-including-covid-19
If a child or young person has a positive COVID-19 test result they should try to stay at home and avoid contact with other people for 3 days after the day they took the test, if they can. After 3 days, if they feel well and do not have a high temperature, the risk of passing the infection on to others is much lower. This is because children and young people tend to be infectious to other people for less time than adults.
Children and young people who usually go to school, college or childcare and who live with someone who has a positive COVID-19 test result should continue to attend as normal.