I remember that in The Voyage of the Dawn Treader movie they added a little girl as a minor character who was not in the book. I guess this was to make the movie more appealing to children. Generally I don’t think it’s a good idea to add characters that weren’t in the original story. There could be some exceptions since sometimes it’s necessary to have extra people in minor roles. But it would not be good to have major characters introduced that are not in the original books in the movie or TV series. It would not be respectful to the author that wrote the stories. 

I think it's worth a mention that for every child actor, they need at least one double and a stand-in. The more child extras you add, the more chaperones and parent helpers they need.
Also for LWW I'd avoid child dwarfs and giants etc - this would be too confusing visually, a logistical nightmare. 

Then of course, there are children boarding the train in London (both BBC and Walden did that). I wouldn't be surprised if a new LWW film or series did that. 

A bit of trivia: Sophie Cook's, who played Susan in the BCC, two children made a cameo appearance on the train in the Walden. 

I can't think of very many scenes in the Narnia stories where there's any great need for children as background characters. The stories themselves are mostly focused on the children who are the main characters. That in itself is surely enough to show that these stories are aimed at children — that nearly all the central characters are under 14 years of age. You don't need a lot of children in the background to prove to young viewers that this story is meant for them; you need main characters who are children themselves and who are easy to relate to, which almost all the protagonists in the Narnia books are. And as @coracle says, the more child actors you have in a production, the more it costs with stand-ins and chaperones and so on, plus the fact that children can legally only work a certain number of hours per week.

I'm much more concerned that any future adaptations of Narnia should get the main characters and the general feel of the stories "right" — that is, so true to the original stories that they feel like the same thing — than whether or not they use children in the occasional crowd scenes. That's a non-issue for me in comparison with those more important things.

Also, ideally a Narnia adaptation shouldn't need to "show that Narnia is a story mainly for children". Everyone knows that the books are children's classics (they've only been around for 70 years or so and LWW, at least, still regularly features in lists of the best children's books of the past century). The bigger challenge is to make screen versions that are suitable for young viewers but also enjoyable for their parents or other adults who aren't "a jolly sight too keen on being grown-up" (unlike Susan) and who understand that a really good story for children has just as much value for adults whose hearts are in the right place, too.

Fair enough. Yes, Narnia is a children's story, but I wouldn't want the movies or series to be made "too kiddy." It should also be made that both children and adults can enjoy.

Then there's the concern how a movie or series would address Susan as no longer a friend of Narnia. I would be concern if they try to change her story, though. We might see maybe Peter, Edmund, and Lucy or even Eustace and Jill trying to get her to come to the gathering of the Friends of Narnia, and she says, "Oh, what memories you have! You're still thinking about those silly games that we used to play when we were children." I can almost picture Polly (who is older and wiser) saying, "I wish she would grow up!"