I think you may be misinterpreting here. The narrator says, in Prince Caspian (Chapter 1),
While [the Pevensie children] were in Narnia they seemed to reign for years and years; but when they came back through the door and found themselves in England again, it all seemed to have taken no time at all.
That, as I interpret it, does not mean that time stopped "in the real world" while the children were in Narnia, but simply that the children's "real-world" re-entrance time was the same as their "real-world" exit time.
That being the case, all that the "time does not pass in the real world" means is that fundamentally, time in Narnia is disconnected (at least in the large scale) from time on Earth. As Edmund says in Chapter 3:
"You know what we were puzzling about last night, that it was only a year ago since we left Narnia but everything looks as if no one had lived in Cair Paravel for hundreds of years? Well, don't you see? You know that, however long we seemed to have lived in Narnia, when we got back through the wardrobe it seemed to have taken no time at all?"
"Go on," said Susan. "I think I'm beginning to understand."
"And that means," continued Edmund, "that, once you're out of Narnia, you have no idea how Narnian time is going."
Which means, then, that if the one child (Lucy) enters, and then the other three after her, there's no necessary connection between when they enter and when she enters—but it doesn't mean that they can't do anything because "time is stopped".