I teach guitar for money and I'm having minor success, so uh here's my advice:
1) Before they learn a single song, get them to learn their chords (Full list of what I teach my students: E, Em, G, A, Am, C, D, Dm and every relative seventh). I know you said you want to leave it up to them but honestly they deserve a firm foundation, plus it helps you teach them later on. As well, show them how to read chording sheets.
2) After that, teach 'em tabulature and all the associated techniques - hammer-ons, taps, stuff like that.
Those two easy steps should supply you with about two months worth of lesson plans if you draw them up in a manner that gives the kids both time and fun. While you're teaching them those basics, throw songs in. Find out what they like and supply them with chord sheets and, later, tabs for them.