In a small bowl, melt candy chips by microwaving for 30 seconds then stirring and repeating until melted.
Add pumpkin puree and stir to combine well.
Spread cereal on lined tray(s) or cookie sheets and pour pumpkin mixture over cereal. We usually need a few trays.
In a bowl, add pumpkin spice to the powdered sugar and mix well.
Sprinkle powdered sugar mixture over the cereal and toss to coat well.
Leave on tray(s) to dry out for a while.
Add M&M's, and toss to mix, when you transfer to a serving bowl.
If making ahead, keep snack mix in an airtight container until use.