The woodland trust is a good place to start. They have loads of downloadable activities for free and you can order trees and seeds from them, all free of charge.
Contact all the major seed suppliers to ask what discounts you get. Also ask them what supporting material they use or provide.
Things that grow quickly are liked by younger kids. Also, things that grow huge. Sunflowers, pumpkins, nasturtiums, beans etc. I always find that these get the kids interested and are a good way to introduce them to watering and planting.
It helps if the kids/parents fill in a simple form when they start telling you what they like, what their growing conditions are and how much space they have. Then you can tailor your classes to this information.
Wiggly wigglers do a great wormery where the worms are sent to you through the post. Again the kids normally love taking them out of the envelope and putting them in the compost heap or wormery.
Mushrooms grown on straw are fun to do.
Butterfly gardens are easy and look lovely in the summer.
Ladybird hotels are really simple and you can paint them with outdoor paint to brighten up the garden.
Bird feeders and tables.
One thing I would ensure is that all your plants are entirely edible and will not cause allergic skin reactions when picked etc. (This is obvious advice but lots of people don't realise the impact of what they plant.) Have fun.