Two chord songs (I use them for beginning ukulele!):
Polly Wolly Doodle is great because you hold each chord for so long.
Believe it or not, so is Hank Williams' Jambalaya- I stumbled upon it by accident and the kids love it!

Buffalo Gals when they get a little faster changing between chords.

I have a long list at school; I'll check it next week and get back to you if you still need ideas then!

Allison