Show different shapes of leaves. Discuss those shapes and the colours those leaves have in autumn.
Kids draw with white crayons different leaves on their drawing sheet. When finished, they tear parts of the different colours of tissue paper (not too small). Use autumn colours like orange, red, yellow and brown. Those pieces must be sticked by wetting the drawing sheet part by part and laying the tissue paper pieces in it. Watch out: no two same colour pieces next to eachother. Be sure the tissue paper is wet enough to bleed.
Let the artwork dry a little. When it's still moist a bit, pull of all parts of tissue paper. Wait until your work is totally dry and press it flat by laying it under a heavy book.