Actually, I do believe they could have been missed. For sure. Kids are small. Kids hid themselves. Canadian wilderness is BIG. Kids that are not found during initial searches are often either out of the search area (we underestimate them) or simply missed. All it takes is one searcher doing one mistake. And IMO while everything else is less likely to happen to two siblings together (murder, accidental death, abduction etc), the one thing that seems more likely is running away (for purpose or accidentally during play). It is not my main theory for timeline reasons, however, I am unable to rule that out. Especially considering the bootprint: SOME little kid clearly was on the pipeline trail. If it wasn't Lilly and Jack, who was it? It's not like it's a popular place to spend time with family.
Yes, it really is super rare. I can recall
one case of young twins, where we suspected mental ilness of the mother (and she indeed is in a psychiatric hospital soon after). It is similar in the sense that the kids were home with an illness, they roled CoD suffocation, but frankly, I think that case had a very different vibe. Also, paramedics were called to the kids, the kids were never missing. I am not saying L&J were not killed by immediate family, it remains one of my top theories, but for now, I have seen no hints about neither the method nor the motive nor the culprit.
A vehicle accident does seem possible for seriously hurting two kids at once, especially kids of a similar age who play together. I find it hard to believe there would be no evidence of that, but if there was relatively little blood and the bodies were moved quickly, then maybe. But I feel the injuries of both kids would have to be pretty obvius (=blood) for the ambulance to not even be called by either parent.
I must say, this really is one of the cases where everything seems kind-of-possible and nothing at all seems likely.