My kids needed a bit more work on C and G, before we add the F chord into the mix. I decided to introduce Buffalo Gals for it's ease of learning, simple chord changes, and it's timelessness.
I introduced the short melody today and next class the kids will sing and play with the chord changes.
This free download includes a picture chart of the C and G chords and then has slides that include the lyrics to the song, with a pictoral version of the chord above each word when a change is required.
Hope this helps your kids improve their chord changing chops.