Slightly misleading to suggest that those children are all "missing". The article suggests that this number includes all children absent from school, including those who are known to be absent for a reason.
The biggest reasons are inability to meet the needs of pupils with ASN and those with ongoing mental health concerns. These are not issues that can be overcome simply by having schools open. Teachers are not social workers, mental health professionals, and most of them have limited training in helping children with ASN.
I don't doubt that there are some children who have been negatively impacted by the closure of schools to most pupils, but other existing reasons for absence have been evident for many, many years.
How many of these children are actually "missing" and what is being done to locate them? That is not an issue for teachers to solve, or even schools to a certain degree.
Underfunding and not supporting vital public services has caused this, not school closures.