One thing that will be very interesting to see is how Netflix chooses to film the seven movies/series. (Not the same as release.)

Aside from potential minor conflicts with the actress who plays Jadis, there is no reason MN and LWW can't be filmed simultaneously. Then you have LWW -> PC -> VDT -> SC -> LB with the Pevensies, Eustace, and Jill.

But HHB is a bit of a wild card because that's the oldest age given for the Pevensies per Lewis's timeline. That and the end of LWW. 

If they choose to go with two sets of Pevensies (for the end of LWW and HHB), then they can film HHB any time they want. But if they decide to just have one set of Pevensies (movie magic for end of LWW), then HHB will need to be filmed last. And at that point, you'll want them to age as much as possible. They don't have to be as old as is suggested on the timeline, but certainly old enough for Susan to be courting Rabadash. I'll also mention there's no way they'll leave High King Peter out of it.  

I'm personally open to ageing them up a little bit. I think personally having actors that fully represent the ages of the characters in the books will always be problematic when working with live-action.

Although as for suggestions for casting the Pevensies - I only have one. Beau Gadsdon.

https://www.imdb.com/name/nm7917803/

She was born in July 2008 so will be 13 later this year, which would currently match up perfectly with Susan, but I do think Gadsdon still looks so young and I can't imagine a Susan looking that young, especially when her character has so much maturity and has to come across older than Lucy - and I can't imagine Beau Gadsdon looking after a younger sibling.

But regardless of age, she is a great young actress, who's appeared in The Crown, and Rogue One.

These are my thoughts in only the book "The Lion the Witch and the Wardrobe" 

I think that Lucy should be a small child maybe seven or eight because as @rose said the reaction and the belief just won't work unless she is a small child. It would also make sense because if someone younger than you says something unbelievable you are less likely to believe it because small children are known for play make-believe with things that aren't there. I would also assume it would make her encounter with Mr. Tumnus more realistic. After all, what child older than 9 or 10 would follow a stranger into the woods to their house?  

I think that Edmund would be a little older than Lucy maybe two years about 10/11. It would help with the scene with the White Witch because for the same reason as Lucy, it would most likely be that they  would be young and naive so it is easy to trick like that. Edmund is a different story though because the White Witch did not only trick him with his age but with his weaknesses and vulnerability. The reason I chose him for 11 also is because of his rudeness in the beginning of the book to Lucy. He would have to be at least a few years older than her to sound believable to Susan and Peter when he claims Lucy made it up. He would also have to be equally mocking to Lucy because it would hurt more if someone close to her age that maybe she thought was going to understand, but then betray her. 

I think Susan would be at least 13. It would make sense for Susan to be 13 because any younger than that would probably not be the best person to handle with a bow and arrow. (I don't know why they gave Edmund a sword) She would also be 13 because she is a responsible older sister. She would need to be older than Lucy, a little more than a few years to have Lucy gain a sense of security in her. All the same she is most of the time making decisions with Peter, making her seem less like Peter's younger sister.  She is always protecting them with Peter and takes on a Motherly role. Not completely but somewhat because she is always wanting to do the most practical choice. 

I think Peter would be about one to two years older than Susan maybe about 14/15. I think that because like Susan he takes on the fatherly role for all of them. (except for Susan) He acts like he is in charge which makes me think he is 15, a little more older than all of them. It would make sense that in Prince Caspian then that he turns 16, going into adulthood (18). It would also make sense because at the age 13, you are probably not even close to ready to lead a battle, let alone be prepared for one. I think the closest you could get to that age would be 15, being older but not to old to be classified as "grown up". I also think he would be 14 because that would make him more of a child than an adult, which would explain why he is not used to being responsible for these kids, because he is not responsible yet. It would also explain finding Aslan as a father figure rather than a friend figure, because all the responsibility was laid on him as a child when they had to leave home in the beginning of the book. He could be 14 because it would explain his closeness of age to Susan and how he lets her make decisions also. 

@kingedthejust I don't know if you saw earlier in the thread, but Rose-Tree Dryad shared this link: Narnia Character Ages

Going by Lewis's own timeline that he put together after writing the books, plus some internal evidence from the stories themselves, we do have the exact or approximate ages of nearly all the main human characters throughout the Chronicles. In The Lion, the Witch and the Wardrobe, when they first enter Narnia, Peter is 13, Susan is 12, Edmund is 10 and Lucy is 8 — so that's pretty close to your estimates.

I'm hoping the people working on Netflix's Narnia will be aware of this too, and cast the characters so they look and act about the age they're meant to be in each story. As you say, Lucy's role in LWW in particular won't work if she's much older than about 8.

I wonder if Walden's excuse for aging up the Pevensies and Caspian was that Caspian and Susan could have a romantic relationship. 

I am not oppose to age up the children. It would be if they had a good reason.

Posted by: @jasminetarkheena

I wonder if Walden's excuse for aging up the Pevensies and Caspian was that Caspian and Susan could have a romantic relationship. 

I am not oppose to age up the children. It would be if they had a good reason.

I think Walden's excuse would be a perfectly valid one - that they didn't.

Georgie Henley was 8 years old when they started filming LWW (the exact right age) and was even younger when they originally cast her.

It's hard enough casting very young children for an acting role in a major production, and your hands are tied by all sorts of employment laws about working with children... But the biggest problem is that it can take several years between casting and production, and then another few years between production and release, and then another few years to spin up a sequel.... all by which time your children have aged 5 or 6 years.

EDIT: in fact, here is a direct quote from Anna Popplewell that sums it up nicely...

"i was 13 when I had my first audition for Susan; 15 when I made the first movie; 16 at the end of the shoot; 17 when it was released; 18 when I filmed the sequel, Prince Caspian, and now I’m 19." 

- Anna Popplewell, May 2008

https://www.dailymail.co.uk/home/you/article-1022168/Anna-Popplewell-lion-8217-s-den.html