Please see latest guidance regarding Coronavirus (COVID-19): self-isolating and close contacts.
For School aged children (including children of primary school age) and young people who are not fully vaccinated... https://www.nidirect.gov.uk/articles/coronavirus-covid-19-self-isolating-and-close-contacts#toc-5
Full article .... https://www.nidirect.gov.uk/articles/coronavirus-covid-19-self-isolating-and-close-contacts#toc-2