I disagree.
First, the actors that play these "kids" are oftentimes older anyway, so that compensates. We have 22 year old actors playing 15 year olds, so I don't think getting a 15 or 16 year old to play a 12-14 year old is too out of line.
Second, people spoke with their wallets after Caspian. They said outright: "You departed from the novel too much, and that killed our interest." Some said "never again." Some said they'll give it one more shot. A lot more likely have said they'll rent it and see if it's worth $1 or $15 at Walmart. If Narnia is to make money, they need to do what the first movie did. Stay closer to the story line. Enhance only where it enhances along the story line. Get in churches and pay tons of money to churches for advertising space. Pay for pulpit access and preacher recommendation from the pulpit.
As much as I (not being a christian) loathed the use of church for marketing purposes, placing Narnia as the "anti-Potter" in the deep south and other areas was brilliant marketing, and led to a ton of money. They need to go back to that method of advertising in the churches that they used before. That's your core audience. That's who needs the marketing. And you need to get them for multiple Sunday mornings and riled up against Harry Potter, or some other evil of the world.