The character of Lucy was named after one of Lewis's godchildren, Lucy Barfield. The idea of the children as evacuees during the war comes from children who were actually evacuated to Lewis's house during WWII. I'm not aware that any of the other children (or even Lucy, save for the name) were inspired by Lewis's other godchildren.
The only other godchild of Lewis's I can think of off the top of my head is Laurence Harwood, son of Lewis's close friend Cecil Harwood.
A quick Google search doesn't turn up a list of godchildren, and I'm at work, so I don't have access to my books. Anybody have a list?