It's almost like playing an acoustic guitar, but with extra buttons and cords. You can play chords on it, but it's mostly played as a solo bit. I've played with one of them before, and I can tell you they're less difficult to press your fingers against the strings than the acoustic
If you don't know how to play any chords, you can look them up on Google under images, that's how I learned how to play guitar.
Sorry, not much advice, since I'm just an acoustic guitar player
I personally like the Yamaha psr-e343 due to the good modern look, but aside that, both don't have much differences. They both have songs that your little cousin can follow through and learn (that's how I first learned Fur Elise with my right hand!).
Proud of your little cousin for having an interest in playing the piano!