According to an article on the CS Lewis Institute website:
The actual wardrobe that prompted the stories was one made by Lewis’s
grandfather and was in the family home in Belfast. Later, it was moved
to Lewis’s home at Oxford and now resides at the Wade Center, at
Wheaton College. One of C.S. Lewis’s cousins, Claire, remembered
occasions when various cousins along with “Jack” (C.S. Lewis) and his
brother Warren, would climb into the wardrobe while young Jack would
tell them stories he had invented. It is interesting to note that
Lewis mentions a few times that “it is foolish to shut oneself into
any wardrobe” perhaps because he always kept a crack of light when he
told his stories and also because he was warned. When Lewis sent a
draft of LWW to friend Owen Barfield, Barfield’s wife Maud was
concerned lest children read the story and accidentally lock
themselves in a wardrobe. So Lewis added five warnings to LWW. The
wardrobe is such a vivid image that one Oxford boy, after reading the
book, chopped a hole in the back of the family wardrobe trying to get
to Narnia.
Note that the Barfield's daughter Lucy was Lewis's goddaughter, and Lucy Pevensie is her namesake.
This information seems to originate from the book Into the Wardrobe: C.S. Lewis and the Narnia Chronicles by David Downing.