Free learning resources for teachers from the RHS:
https://schoolgardening.rhs.org.uk/home
Free seeds:
https://home.38degrees.org.uk/2023/02/07/seeds-for-bees-frequently-asked-questions/
About halfway down the page, you'll find a link to request additional seeds for schools and community projects. The selection includes a variety of easy-to-grow plants, perfect for children, especially the sunflowers.
Free trees:
https://www.woodlandtrust.org.uk/plant-trees/schools-and-communities