Here are a couple of more ideas.
Insects: what is an insect?
Bees and pollination
Other beneficial insects
Harmful insects
Predators of insects and how we can attract them to our garden
Tools: types of garden tools and safety
Care and maintenance of tools
How each tool is used
I've used variations on these from grades K-1 on up and have always used the
"tecnical" terms for plant parts, Pistil, stamen anther etc. Even the little
ones remember much. Also, most of these topics can be classed as science,
langauage arts, spelling, for those who ask why you are spending time on
"gardening" when there is "real" learning to be done!!
(I'm sure you know what I mean)
Have fun with it
Emilie