Probably because panic sets in first
The adrenaline rush when you're in panic. This happened to my cousin to be honest, I think ours was worst cause it took 2hrs for us to find him hiding under the bed. All of us adults was in panic not thinking of this simple places for them to hide.
Maybe because its where they realize Minato was missing. Hazuki thought that Minato go back to his Mom and vice versa. And because of Minato was not in the room (or bed) with Hazuki, they search to another place.
Y'all under the bed is such a standard place for a little kid to be sleeping why didn't anyone check there first