There are many rock songs with two chords-- Iko,Iko (a song kids love!) Dream Baby (which many kids know from a recent tv commercial) Rockin' Pnuemonia (and the boogie woogie flu) Bo Diddley (which just a funny version of Mockingbird)
Lots of songs that don't sound simple have only two chords as well, such as "Just my Imagination"
Many rap tunes can be played with a single chord accompaniment--or may even with just a clapped rhythm--you say you don't know any rap ? The kids do--and they'd love to teach you(make sure the lyrics are safe first though)--
If you find songs that are interesting and fun, the kids can really open up, andyou will really have a great time--