I know, as a police officer of many years, that the threshold to take a missing person report is for someone to report someone else missing and have no clue as to their whereabouts.
It may surprise you to know that missing person reports are filed for teenagers that have stayed out past curfew, for men that have went on work nights out and stayed out all night without telling wife where they are, the threshold is surprisingly low, the only things we need to have are a person reporting that as far as they are concerned, another person is missing, and us not being able to immediately clarify that they are not.
As an adult, I could send my parents a text message and say I’ve decided to move elsewhere and want no further contact and I’d bet much be classed as a missing person if they decided to report me as one to police.
I don’t know, and neither does anyone in this thread, exactly what OPs brother has told police regarding the circumstances, I do know that some cops will do anything to avoid a bit of hard work if a loophole allows but “we” would very much be obligated to class her and the kids as missing.
After all, we can’t actually say for certain that she, or particularly the kids, haven’t come to harm, this needs to be followed up. Because if it isn’t, we risk a horrendous situation where mother and/or kids could potentially have come to harm and nobody would ever know. That’s why a cop needs to lay eyes on them all to confirm welfare.
incidentally, depending on how the circumstances were described to officers when OPs brother contacted them, it’s not a huge surprise that it was brushed off, what he needs to do OP is to say that ex and kids have disappeared into thin air, no clue as to their whereabouts, no forwarding address or school details, he has huge concerns over their welfare/wellbeing.