This is a terrible thought but what about their father, Jim Beaumont as a suspect. It's hard to kidnap three small children. One of them is bound to struggle or scream or kick up a fuss and attract attention. The man who took them must have been someone they knew? Jim Beaumont had been a taxi driver but around the time of the disappearance he was away on a business trip but told police his clients never showed up. I would like to know more about this but cannot find the information. What kind of business was he in since leaving taxi driving?
Jim was the first one to report to police the kids were missing. He also stayed close to police investigations. OK he was the father, but if he had been the perpetrator as well maybe he stayed close to the police investigation to keep up with what they were discovering or even to mislead or derail the investigation?
I can't think of a motive for him to kill his kids but stranger things have happened. Would love to know more but it's so hard to get info on such an old cold case with no bodies.