The information comes from E. Milby Burton's book The Siege of Charleston 1861-1865, pages 9-11 of the paperback version. It is the only place I have seen it.
On the afternoon of December 26, the schooners with the women and children left the dock at Moultrie under the command of Lieutenant Hall. Not until then did Major Anderson reveal his plan. He instructed Hall to delay unloading his personnel and cargo under the pretext of finding suitable quarters at Fort Johnson. When he heard two guns, Hall was to come at once to Sumter.
... When the last man from Moultrie was on Sumter, the two guns were fired, signaling recall of the schooners waiting at Fort Johnson with the women, children, and supplies aboard. The captain of one of the schooners, realizing that he had been duped, put up a fight and had to be overcome by force.
You can rest assured that I don't plan to overcome my cab driver by force and take his cab where he had not contracted to go.
Here are some comments on the back cover of Burton's book:
"Burton's book will appeal to armchair tacticians interested in ironclads, torpedoes, submarines, and stories of 'courage and honor.' Historians familiar with the major events of the Civil War will welcome this comprehensive and generously documented book ..." [Journal of American History]
"The author, the director of the Charleston Museum and a retired naval officer, has brought together, mainly from original sources, more information on the siege than is to be found elsewhere." [American Historical Review]
I purchased my copy of the book in Charleston at the Park Service bookstore at the dock where boats depart for Fort Sumter.
That's a slightly different spin than what you described the first time.