how to make the child always look down ?

how to close so that the child always looks down, even if the parent makes any movement to the right, left, turn, the child must always look down

If the parent only rotates around y axis, then the problem is solved by setting the initial orientation of the child facing down since movement of the parent will not change it.

Otherwise:

a) Child object dosent really need to be a child of the parent:

You can just put a script on the “child” object which will in the Update synchronize the position with the parent (Do not parent it to the parent object in this scenario). This will only update child position but not the rotation.

Transform parent;

void Update
{
    transform.position = parent.transform.position;
}

b) Child object really must be a child of the parent object:

You can add a script to the child object which in Update will be changing the orientation of the child object so that it looks down all the time (In this scenario keep the child object parented to the parent object.)

void Update
{
    transform.LookAt(transform.position + Vector3.down);
}

Didnt test the code but I think it should work.