Our Sunday school decided to make artworks to give to Allowah, a hospital for children with disabilities.
I wanted to create an artwork that all the kids in our preschool class could contribute to, but would also look nice. So I got a canvas from a discount store and painted a background and a tree trunk on it. I then painted the kids hands and used their hand prints to create the leaves of the tree. I really like the effect.
I think you could use a similar idea to make a card; draw a simple tree trunk on cardboard and get children to make a green hand print above each.