BTW,there is actually no way to know that. They might not be mandated as a result of their type of work, but in some locales, all residents are mandated reporters...or they could, for example, be foster parents or otherwise be mandated reporters for non-occupational reasons.
That said, I am the parent of a couple of children who were/are runners as little ones, and I understand that the little monkeys can get away when you least expect it. I probably wouldn't report a situation like the OPs. IMO, no harm, no foul.
A couple of years ago, at the school, we had an incident where a 2 year old girl walked 3 blocks from her home up to the school, which required crossing 3 streets. One of the staff members found her wandering around the campus. The office manager happens to live in the neighborhood, and recognized the child, called her older brother (a first or second grader) up to office to keep his sister company, and called their mom. Their mother wasn't answering the phone, so the police were called, and one of the staff members drove down to the child's house and found Mom looking frantically. IIRC, nothing came of it, except that now that Mom knows that her younger kiddo knows the way to brother's school. LOL