Local authorities have a duty under section 436A Education Act 1996 to make arrangements to identify children outside school who aren’t receiving education.
436A became law in Wales in September 2009 (see Commencement Order here) The GOV.WALES page on Children Missing Education [CME] is here https://www.gov.wales/statutory-guidance-help-prevent-children-and-young-people-missing-education
The Law
436A Duty to make arrangements to identify children not receiving education
(1) A local authority must make arrangements to enable them to establish (so far as it is possible to do so) the identities of children in their area who are of compulsory school age but —
(a) are not registered pupils at a school, and
(b) are not receiving suitable education otherwise than at a school.
(2) In exercising their functions under this section a local authority must have regard to any guidance given from time to time by the Secretary of State.
(3) In this Chapter, “suitable education”, in relation to a child, means efficient full-time education suitable to his age, ability and aptitude and to any special educational needs he may have.”
Guidance
Statutory guidance to help prevent children and young people from missing education in Wales can be found here https://www.gov.wales/sites/default/files/publications/2020-09/statutory-guidance-help-prevent-children-young-people-missing-education.pdf via this page https://www.gov.wales/statutory-guidance-help-prevent-children-and-young-people-missing-education
Who is CME
Unless they are being educated otherwise than at school, children are missing education [CME] if they are of compulsory school age and don’t have a school place. This is often as a result of needing a place outside the normal admission round because the family has moved out of the catchment area of the old school.
Sometimes children fail to start school at all; an application is made but then the child does not attend, or parents don’t get their preferred school. A child might also lose a school place by being permanently excluded and will be CME if alternative arrangements are not made.
A home educated child may become CME after it has been “considered that home education provision was not suitable” ie after the LA has not been satisfied and has reached a definite negative decision. [Source = CME Guidance p 92]