I once (ages ago) drafted a letter that I was going to send to Walden, but I decided against it. I do think it sums up my response to the OP's question, though:
The Silver Chair has so much potential as a feature film. It has a Sleeping Beauty quest-like story, except that the “beauty” is a man and he is rescued by a girl (one of the things I love about Lewis is his proto-feminism). The characters are colorful and the kids are extremely relatable. With the right pacing, script and visuals, it could be a movie that many movie-goers would enjoy, and come back to see again and again. In fact, I think the story could work a lot better on film than it does on the page. I am not trying to sell a script, I’m only trying to give you some general ideas about the book’s cinematic potential. This could be a truly great movie, but certain pratfalls must be avoided.
First of all, know your audience. The Narnia books are family entertainment. They are magical enough to engage elementary-schoolers, but intelligent enough to engage their parents. Your The Lion, the Witch and the Wardrobe film struck this balance perfectly. Prince Caspian was, in my opinion, a good movie, but it was far too teen-centric to appeal to the elementary crowd, who are a critical element of the Narnia fan base. A Narnia movie must have that magical fairytale feel to it.
How does that apply to Silver Chair? Capitalize on the moments of brightness and magic: Aslan’s Country; the wonder of discovery for Jill (remember she’s as new to Narnia as Lucy was in LWW); the merry court of Cair Paravel; the crisp, invigorating land of the marshwiggles; the enchantment when they first meet the Green Lady (they think she’s benevolent and lovely in that scene); the bustling court of Harfang when they first arrive; the glowing forest of Underland (I think your visual effects people could do an amazing job with that); the bejeweled land of Bism, perhaps; and the snow dance. Also, make sure the moments of darkness have a feel of suspense and adventure, like the dark moments in LWW, rather than the period-piece gloominess of Prince Caspian. The fight with the Green Lady would need to be reworked somehow, so that the movie doesn’t devolve into horror.
As I said before, the Narnia books are both magical and intelligent, thus designed to appeal to all members of a family. Voyage of the Dawn Treader had the fairytale magic down pat, but with over-simplified themes and dumbed-down dialogue, it was more of a “kiddie” film, like the Tinkerbell or Barbie film franchises, rather than a family film. Kiddie films typically go straight to DVD and even those (like Yogi Bear) that make it into theatres never make enough to justify a block-buster budget. There’s a reason for that. Kiddies don’t pay the price of admission, their parents do.
How do you make Silver Chair intelligent enough to satisfy the parents? Remain faithful to the thematic complexity of the book, for one. Of course, you should trim some of the visually duller sequences and embellish some of the more interesting but, unlike Caspian or Dawn Treader, broad changes are neither necessary nor helpful. For another, although some dialogue needs updating for audience comprehension, keep a healthy helping of Lewis’ dialogue in there, especially when it comes to Puddleglum’s lines. He’s hysterical.
So much for the target audience, now on to marketing. The American marketing for VDT made it look like a feeble attempt to milk what was left of LWW’s good will, rather than an interesting movie in its own right. Any future Narnia movie has to be sold on its own merits. If it wasn’t for parents scouring the papers for G/PG films to entertain their little ones over the holidays, VDT would likely have fallen off the map by its second week. In the case of Silver Chair, it would probably help to drop the “Chronicles of Narnia” from the title, since people tend to look askance at sequels that don’t feature any of the original protagonists. . . .
Silver Chair. . . must be marketed as a stand-alone. I’m not saying you should hide the fact that it’s Narnia, but let audiences understand that Silver Chair is its own story, not a follow up to the Pevensie trilogy. Sell the characters. Jill and Eustace are a couple of boarding school misfits who learn to like and trust each other in spite of their differences and initial mistrust. I think it would be best to portray them as middle-schoolers, aged 12 or 13. In marketing Silver Chair, you should emphasize the fairytale adventure too, but the character dynamics are even more important. Those dynamics are what set this story apart from generic mid-grade fantasy.
I think a well-marketed, well-written adaptation of Silver Chair could bring in a lot of casual fans and capture the imaginations of the movie-going public. It might even leave people clamouring for a Silver Chair 2.
and keep the characters true. 
and that I can't wait to see it. 
