Pumpkins don't transplant well. It sounds like you picked the biggest plants to move and that's where you went wrong. You want to try and move the smallest of the plants since they have the best chance of survival. And when I say small I don't mean thin and spindly I mean the fewest leaves. Try and move ones that have only their seed leaves and take as much dirt around them as possible. If they are all clustered together don't pull up the plants around the one you want to transplant as this will disturb the roots of the one you want to save, instead you should cut them off at ground level. If they are all big your best bet is to just leave one in the compost pile if possible.

Thank you for such good info!

Do you know of any way to tell pumpkins apart from cukes/melons, esp. in that early stage? I had to wait until they were huge before I realized they were pumpkins.

It is hard to tell them apart that early. I can tell you that sometimes the seeds stay attached to the seed leaves when they sprout (or are laying on the surface of the soil) and you could try and tell from the seed if it is present. Watermelon have black seeds, cantaloupes have pointy long seeds. Cucumbers and pumpkins/gourds have very similar seeds and are hard to distinguish. However, most (if not all) cucumbers are harvested when they are immature and the seeds aren't fully developed. This means that you should almost never have cucumber seedlings. Also large carving pumpkins have larger seeds than cucumbers or ornamental gourds. And the hard shelled gourds (with the white flowers) have dark, somewhat rectangular seeds.

Hope that helps.

Just to follow up...one of those transplanted pumpkins has put out new growth. IT LIVES.