Even the smallest child can make something of great beauty with this collage technique.
Cut a picture frame out of cardboard, sizing it so your child can comfortably carry it.
Turn it plain side down, and measure out enough clear contact paper to stick all around the frame. (Some people call it shelf paper instead -- you can find it in the housewares section of a big store.)
Peel off the backing, stick down the contact paper, turn it over, and voila! You have a sticky canvas for your child to decorate.
You could use this technique for any sort of collage, of course -- it needn't have a nature theme. But I love the experience of accompanying my kids as their take their little picture frames along on meadow walks or strolls the the woods. I find it helps them slow down and really look at the natural world around them.