Since pumpkins are organic material, I'm assuming it would be okay to add them to a composting pile.
Is there any reason as to why pumpkins shouldn't be composted?
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Pumpkins are easy to compost. If they were used for crafts/decoration, it's possible that they contain inorganic matter such as paint, ribbon, candle wax, plastic twine, foil, etc. Make sure all such material (if any) is removed. The seeds will survive all but the hottest compost heaps, and can be a nuisance later. I don't usually worry about it, and pull pumpkin seedlings with the weeds, but you could roast and eat them, save them for next years seed (they may be from hybrids, so don't expect much uniformity), or run them through a food processor.
Pumpkins are a green (high nitrogen) material, so leaving them whole will cause improper initial decomposition, and you may lose valuable nitrogen content by evaporation and anaerobic decomposition. The ideal way to go about this is to chop them into small pieces, the smaller the better, to increase surface area, and mix/layer them well with carbonaceous material. Put a layer of carbon/soil over the top to hold in moisture, and to keep insects off.
The stem and the rind are the parts that take the longest to break down (excluding the seed hulls), so again, chop everything into as small of pieces as you think is practical.