Imo, I already think it's too expensive for the age group their targeting (7-17) though. But I agree with you that instead of just starting them with a proprietary text editor that nags you, it's better to educate about licensing beforehand. Otherwise, it'll just perpetuate the notion the 'nagging' dialog doesn't matter, until they found out when it's probably too late (as in they're already locked in due to habit).
What if they agree that free software is ethically better? They might not (they probably don't care), but shouldn't we at least tell them what they're getting into just in case?
1) developers are not paid accordingly to their contributions and I'm thinking here especially of indie games developers that suffer the most from this
2) open-source is not as popular as it should - for example whenever a Gimp vs Photoshop thread comes up in which Gimp is said to "suck" by comparison, it infuriates me immensely since I bet that a large percentage of Photoshop users are pirating it and so Gimp doesn't get the credit or the attention that it deserves. Actually piracy limits competition, since this is a problem for all Photoshop alternatives, not only for open-source.
3) monstrosities like DRM which end up punishing well behaved people, since DRM doesn't really work except for lock-in purposes, but hey, companies get away with it because piracy
4) closed platforms that mandate the installation of apps from app stores, because safety and piracy
In general I do not agree that piracy is like stealing, but it does real harm and children should be taught not to pirate, primarily out of respect for other human beings.
There is a problem however - children need to be educated in matters of licensing, which is why I believe starting with an open-source editor makes sense. The real power with open-source is that as a developer you've got control over it and that it never dies. We wouldn't even have this discussion if we were talking about a proprietary programming language with no open-source clone.
So there's a risk associated with picking something proprietary - those risks are sometimes worth it, Sublime Text is surely a good editor and much friendlier than Emacs and if it dies or gets too expensive, then the lock-in is not that great, but children at some point need to learn about licensing and the pros and cons of the choices they make.
But no, I don't think it's unethical.