The rotary tools do not work well for this. The pumpkin parts just clog up the bit. It is actually amazing how poorly they work.
Like cowboy73 said, a jigsaw works ok but is kind of bulky to use around kids.
I want to try out my Dremmel Multi Max this year and see how it does.
I have used a drywall knife in the past and it has worked great. Lots of big teeth and a nice handle make the work quick and much faster than those little knifelets that come in the kits.
Edit: Here is a link that backs up what I am saying....
http://www.extremepumpkins.com/whattooldonw.html