Don't you think that river or stream nearby loomed so large in everybody's mind, fearfully so, that it could have been why the car was only cursorily searched? Also, how did the kids get the trunk lid down by themselves? Inside handle?
You can pull a trunk lid down from the inside, it's not difficult at all.
Sure, the river would be the next place I would have been checking after I thoroughly checked my property, my neighbors property and anywhere else in the vicinity that they would usually like to wonder to.