I supposed it depends on what you classify as folk tunes/songs. There's a whole genre of music that can be considered folk songs and/or tunes depending on where you live, not just the songs you hear on Sesame Street, or by the likes of Fred Penner, or Sharon, Lois, and Bram, or even Raffi...but those could be a good jump off point for songs to possibly add to your repertoire. Really, folk songs are songs that are usually (but not always!) passed from one generation to the next. They don't have to be old to be considered folk songs - take 'Sonny's Dream' by Ron Hynes for example...it's considered folk by many even though it was written in the past 30 years. Or even 'Any Mummers Allowed In?' (aka the Mummer's Song) - written by Bud Davidge of Simani in 1983 (if my memory serves me right...it's been a long day, so I could be wrong)..it is considered part of the folk tradition by a lot of people too. *okay, taking off my folklore hat now*
I'd suggest for the younger set try any nursery rhymes that have been set to music. I'm thinking along the lines of "Old King Cole" and songs like that.

Don't feed the idiots or they'll continue being idiots...