Why is the collider not working?

Question

Unity

Good afternoon, I created a child object, gave it a box collider, rotated the object as needed, but it doesn’t work, the object just falls through it, and the collider that the parent has works as it should.
I know for sure that I missed some trifle, tell me please, how to be?

Answer 1

All colliders on child objects are bound to a Rigidbody, and this object is treated as one with a complex collider in the physics engine. If you need to create any obstacles, then they must be on separate objects.

Answer 2

box collider to be placed in the main object.