C.S. Lewis and Tolkien were friends and they challenged each other to write a series of fiction novels which would teach Christianity. Tolkien used symbolism in his "Hobbit" book, while Lewis used allegory in his "Lion, witch and wardrobe" books. These books were geared towards children while they also wrote adult-themed books (i.e. Tolkien's "Lord of the Rings" and Lewis' "Perelandra" trilogy).
Tolkien's themes in his books are basic, classic good vs evil with Catholic themes mixed in. They are fantastic reads with good moral themes.
I can say the same thing about Lewis' "Lion, witch and wardrobe" but I haven't read the "Perelandra" series so I can't give it a stamp of approval. However, Lewis is supported by most good Protestants so I think his stuff is ok. Never heard any moral issues with his material at all. The only thing you could say about C.S. Lewis is that he wrote many other books about adult-themed issues (i.e. "screwtape letters" book) which pertain to the devil and aren't for kids.
On the whole, Tolkien is 100% catholic and he was even against V2 when it came out. Lewis is a protestant at heart, whom Tolkien tried to convert his whole life and whose writings show a large catholic influence. Lewis' protestantism is not the kind we see today; he grew up in the 30s-40s so it's much more orthodox than the "God loves everyone" and "I'm already saved" message of today's nutballs. Lewis makes today's protestants seem like pagans.