I'm late to this thread, but wanted to mention that a deep enough sink hole could definitely keep a small child alive far longer than being exposed to the elements in an open area. It would protect from wind and also be considerably warmer than the earth at ground level. A 4 ft. deep hole would be enough to provide some heat benefits. 6-8 ft would be even better as it would provide more insulation and more protection from the outside air. The ground temperature once you reach 3-4 ft in depth will stay at a pretty consistent 50-55 degrees. It would likely be a bit colder due to exposure to the outside air, but definitely warmer than nothing at all. It would also help delay dehydration. The air would be more moist, therefore less moisture lost during normal respiration. I don't know how much survival time it would add, just that it would be beneficial in a case like this. It wouldn't be a fun fall, but it seems young kids are far less likely to suffer major injuries in falls.
How far was the closest road to where they were walking? How far is a road with heavier use like a highway? If he didn't fall in a hole initially, I'm wondering if once it started getting dark, could he have been close enough to a road to see headlights or hear traffic?
Sadly, that's a massive area and I'm afraid the initial searches the first day or so were lacking. I think initially a toddler wouldn't follow the path of least resistance. They'd follow the path of most interest. I think as they wore down, their path would become more logical, such as following a path or logging road or just a clearing of some sort, but I understand wanting to cover the most amount of ground in the least amount of time possible.