That's an excellent topic to discuss! Here's some thoughts...
First of all, let me recommend that you read "On Three Ways of Writing for Children" by Lewis. You can find it at the back of the adult editions of CoN. Another good one to read is "On Fairy Stories" by Tolkien.
I think that it's healthy for children to imagine and pretend, but they aren't stupid! They know the difference between reality and fantasy. It is adults (well-meaning adults, granted) who blur these lines in a child's mind.
When I was a child, we used to pretend that Santa was coming every Christmas, but even the smallest of us knew that we were pretending. We never actually believed that Santa was real, and I think we had more fun than other kids who were deceived by their parents.
I remember as a young teen feeling closer to God through Aslan as well. He just seemed more approachable somehow at that time. As I've matured as a Christian however, I've found that the traits that Lewis wrote which made Aslan feel so near to me are the same traits that Jesus possesses in actuality! And of course, I realized that (mentally) as a child; but I couldn't feel it for some reason.
Why do you say that children can't imagine real angels and demons? For one thing, they don't have to 'imagine them up' since angels and demons are real. They can take God's Word for it and learn the truth.
The Narnian creatures are a good likeness and maybe a useful tool to teach children what is real. You could say, demons are sort of like those bogies and hags in the White Witch's army because they are both evil and ugly and want to hurt us if they can. It's just an allegory though; don't read too much into it.
Maybe you are confusing "belief" and "suspended reality."
Belief is a mental acceptance of and conviction in the truth, actuality, or validity of something. (Per Webster) There's some fact behind it that the faith is anchored on.
Suspended reality is when your imagination kicks in and while you are reading that book or playing that game you pretend that you believe in dragons. If your imagination is strong, then for a moment you practically
do believe in dragons. Your reality is suspended for a moment, and an alternate reality is prominent in your mind. Then you come back down to earth, and you know what is real in the world.
I agree that a kid who says "I don't believe in dragons" is not exactly a fun kid and probably hasn't much imagination; but that's no reason to judge him. Some people just need facts and no more. They find it hard to suspend reality and pretend to believe.
In closing, please please please don't lie to kids and tell them that talking creatures exist! They'll only hate you for it later. Encourage them to pretend and imagine because that will enrich their minds and lives, but don't blur that line of reality.
You may wonder where I get all this from... Years of working with all kinds of children, tough educational pysch. classes in college, and a hobby in children's literature. No wonder I feel rather strongly about the subject! I've seen people from both extremes: those who think kids should never pretend anything that isn't true, and people who think kids should actually believe things that aren't true. Both those extremes are wrong, in my opinion. God tells the Christian to be moderate in all things, to find a balance between extremes.
Just some thoughts...
God Bless!
And welcome to the Wardrobe!
"My brain and this world don't fit each other, and there's an end of it!" - G.K. Chesterton