Okay, so CC-BY-NC-SA is non-free and cannot be distributed in the same package as GPL software. Thanks Zefk for the clarification, I was not aware of the problem.
But it would be terrible to separate the sample quest from the engine and editor distribution (especially not from the editor distribution), because the goal is to provide assets to users who create a new quest.
If we separate it, only a few poeple will use it. If it is non-free, it cannot be included in Linux distributions and will get much less visilibity.
Diarandor, I think you should consider switching to a CC-BY-SA license for your art

First, you told that one day you would want to charge some money for your art. This implies changing the license anyway.
If we put a CC-BY-SA license, not only it will simplify the licensing of the sample quest and Children of Solarus, but more importantly, the art would be free as in freedom, distributed with Solarus: more people will contribute in the end, making awesome games, improving the art again. Just like people contribute to the engine and editor and improve them a lot.
I don't think it is a problem to allow commercial redistributions. If people redistribute existing games for money without modification, it won't work for them, our games are already gratis. If they do some real work and want to charge money, then good for them, and they might as well get something from it. Keep in mind that their buyers will then have maximum freedom, including the right to redistribute for no money what they just purchased.
See
http://www.gnu.org/philosophy/selling.html"Distributing free software is an opportunity to raise funds for development. Don't waste it!"
Imagine that after the Children of Solarus release, someone makes an Android port and sells Children of Solarus on Google Play. Are they getting illegitimate money by doing that? No, they deserve some compensation, this is some real work. But think about it. If it is good, I will totally buy it. Thanks to GPL and CC-BY-SA, I will also get the source code of their work and the right to redistribute it like I want. So we would redistribute it on the Solarus website, and probably on Google Play under the name "Solarus team", possibly for money. Who benefits more? Solarus!