This tried and tested lesson plan is great for two hours worth of ukulele instruction! I taught ukulele to 4th and 5th graders this year (4 classes per grade, about 28 kids per class) using this plan. On the first day you can go over the parts of the instrument and class rules for using it, and teach C and F chords. I taught G and G7 on the second day.
I also recommend using file label stickers to color-code chord positions on the fretboard of each ukulele, if you are able to do that!
The pdf includes the lesson plan itself, as well as the chords for various songs, and a chord chart template to post in the classroom.