Select the child object and hit Alt + P, then Clear Parent (also available from the Object menu.
Documentation is here: https://docs.blender.org/manual/en/latest/scene_layout/object/editing/parent.html#clear-parent
In latter versions there is the ability to batch remove a number of ID objects in one fell swoop.
batch_remove(ids=(id1, id2, ...)
.. method:: batch_remove(ids=(id1, id2, ...))
Remove (delete) several IDs at once.
WARNING: Considered experimental feature currently.
Note that this function is quicker than individual ...