While I would agree that many children at age 5 can be taught how to activate a PLB, I wonder how that translates to the real world with kids who rarely have good judgement skills, especially boys. <sigh> I would be concerned about them deciding to show-off or otherwise play with the PLB, setting it off in a non-emergency situation. That could cause significant problems, not even considering it is illegal. Obviously, as they get older it is less of a problem.

You also have the not unusual fear of getting in trouble that has caused a number of fatalities or near fatalities as young children avoid searchers for fear of adverse consequences to them from parents. That can be somewhat ameliorated by training, but that training has to consistent with how they are treated in other situations as well or it can be ineffective.

If you have cell phone coverage, then there are readily available tracking options, but without cell coverage, something like a SPOT would work for tracking them if tracking is turned on. That would be the best solution, I expect.

Beyond that technology solution, I don't think there's a good non-tech answer to situations such as this beyond being constantly vigilant. It really is no different than kids around water. It only takes a moment for things to go from no threat or fun times to potential death.