If, when a person is in Narnia, time does not pass in the real world and children who visit Narnia enter it one at a time (as in a queue), then after the first child crosses over how do the other 3 enter Narnia as well?
I ask this because after the first child enters, if the time in the real world is stopped then the other 3 children should not be able to do anything.
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".
Simply there is no time correlation between Narnia and Earth. At each instance of a visit to Narnia time flows differently depending on Aslan's personal touch. We have instances where 60 years was a minute and instances of an hour being a minute. We have instances where Lucy enters the wardrobe and minutes later in the real world Edmund also enters the wardrobe that it has only been hours in Narnia. We have instances where they enter Narnia in back to back days and it's only days in Narnia, or with a week span in the real world with 1 year in Narnia. 1 years after The Lion, the Witch and the Wardrobe it's close to 2000 years later. With these inconsistencies we can understand that there is no correlation to real world and Narnia and as such we cannot speculate at all.
