Your post reminded me of a place that someone hid some bodies once and it was so unique that I don't think the bodies would have ever been found if the person had not confessed and directed searchers to the bodies.
He hid the bodies in a hollowed out dead tree. It was just incredible and I think it was the first case of its kind. Matthew Hoffman, an unemployed tree-trimmer led police to the bodies. Link below. This particular tree was a hollowed out tree that had died and the hollowed out area was only accessible by climbing up the tree some and stuffing the bodies in the tree trunk from the top down.
I've been in quite a few woods and trees that could be used for that are not very common but they do exist. I think certain types of trees it is more common than others like Beech trees. It usually happens when a tree breaks in half due to lightning or just dies and breaks in half leaving enough of the base. Then over time animals and decay begin to dig out the inside of the trunk.
If VDK had spotted a tree such as this then I think he could have dumped the little boy inside a hollow tree trunk.
Since the child was small there are probably quite a few trees that could be found like this since the diameter would not have to be that large.
It is such a hideous and unique spot that nobody would ever think to even look there and dogs would have a very difficult time even smelling decomposition inside the trunk because the odor goes straight up the top and out the hole on top. Searchers would not typically think to even look inside a hollowed out tree if the top is only accessible by a little climbing.
I've mentioned before in other cases that people have a tendency to not look up when searching. Its just something that we don't do. Its something someone told me about and it is just amazing that he was right because whenever you go hiking in the woods you just do not look up often at all. Sure you will occasionally take a small look upwards but more often than not your eyes stay focused in front of you or on the ground. It is something you have to force yourself to be aware of and also look upwards. I don't think searchers would know to do that unless they are trained well.
In the case in link below, searchers had to saw down the tree at the base to recover the bodies. This particular case it was a very large tree because the murderer put down multiple people and even their family dog. For a little boy the tree would not have to be that large diameter and I am betting finding a suitable tree would not be too difficult since boy was small. Its just something that I think searchers need to consider when looking for the boy.
http://www.nbcnews.com/id/40275448/...town-grieves-hollow-tree-held-bodies-removed/