I think perhaps the first "marketable" skill would be just how to use a computer.
Forget all the programming stuff - that's cool and all, but do these kids know how to use a computer to begin with?
Explain away the magic. Teach them how to use a word processor, spreadsheet and presentation program. Doesn't have to be LibreOffice or Word or whatever - any generic word processor and spreadsheet will do.
The goal is to give them skills useful for life - perhaps they have a report they need to write - show them how after all their research is done, to type it up, and print it out and how neat and tidy it comes out.
That's a skill they'll need just to move on - and they can immediately benefit by producing homework that they're proud to turn in that looks all neat and professional.
Once they've mastered that skill, then you'll have figured out who in the group is technically minded and wants to do stuff, who needs reinforcement of the basics, and who still is too afraid of the computer. You can then show the technical group stuff about programming, fixing and mucking around the computer. Those who are struggling with the basics you can reinforce and help out with their homework assignments, and those afraid of the computer? Well, show them how to not be afraid. If they're afraid of breaking it, bring in a junker that works and show them that it's really quite hard to break a computer without taking a sledgehammer to it.
Before you even consider jumping into the technical side, see if your group has basic skills everyone will assume they have - typing, how to use a word processor, using the Internet, etc. Only then should you move on.