Each child can use pastel paper in shades of gray or beige, and draw the blossoms in detail with the pencil, then use one color of pastel as a very carefully applied accent on the blossoms. Rousseau did fanciful jungle paintings. His inspiration came from zoos and botanical gardens. Invite children to draw real plants brought to the classroom. Bring at least five different kinds with different leaves and rotate them between the drawing tables so all children can have a chance to see and draw each type of plant.
Children then can use their plant drawings as a resource to create a jungle painting of their own.
Try crayon resist or colored pencils also, especially if you want children to get the details of the leaves. Read The Great Kapok Tree by Lynn Cherry, and draw children's attention to its awesome illustrations of rainforest plants and animals. Bring in some white carnations or daffodils and show how they change color when put in colored water. Teach color at the same time. Follow with color mixing experiments using watered down tempera paints.