1. Is your smb mount read only?
If not, that visiting kid can delete all your media through kodi's file manager.
2. Is your smb mount only providing media access?
If not, that visiting kid can access all the other sensitive stuff on your nas via the file manager.
Those have nothing to do with how the credentials are stored.
If your answer is yes to both, then the credentials potentially being read would be not that big of a deal as that kid can already access all the contents through the file manager.
The only issue I can see is if you reuse the same credentials so the kid being able to see the credentials causes other issues. But that's also a no no. Never use the same credentials for different things.
All in all, I don't really see a significant security issue here. Whoever can see the credentials don't really need the credentials because they can already access those things.
I believe you can protect the kodi interface with a pin (not 100% sure) but in any case, whoever you trust with access to your libreelec box, you're already trusting with access to the media on smb. Make sure it's read only to be safe.