This project would look so great with watercolor. Have your kids wet the watercolor paper before applying the paint to the back of the leaves if you go that way. It's similar to my moku hanga adaptation posted here.
STEP 1 - Collect botanicals! Leaves work very well. Try pine leaves, orange peels, pomegranate, grains, or grass reeds if they grow where you live. Palm fronds would work wonderful, as well as eucalyptus leaves.
STEP 2 - Apply diluted gouache or watercolor (or acrylic if that's all you've got) to the surface of the leaves. If you want to go the moku hanga route, coat them with a thin glutinous rice paste or glue paste first.
STEP 3 - Apply to wet or dry watercolor paper. Turn the side of the botanical with paint facing down, and press the paint onto the watercolor paper surface.
STEP 4 - Dry and display. If you can find a rice starch paper, these work really great for wrapping and giving gifts.
For the young kids make sure to be as kinaesthetic in delivery as you can. Wiggle, squiggle, and bend your body and face if you want them to love what they're doing. Pat the leaf down like you were petting a dog, and bark, "woof". If art is a static picture in time, then the way it gets there is through movement. Make sure your movement is animated, and filled with joy if you want a joyful project. Move slower and more lengthy like you were doing a noh or butoh dance for more somber and reflective things. It's all good, and it's all necessary; every single emotion you and your students have. Turn the bad seeds into fertilizer or medicine. If you've got lemons, get a bee-suit for some honey.