I can't remember where I heard of this idea but I did it last year as well! I showed it at circle time making sure everyone knew to wear the goggles, hold the hammer with two hands, and do gentle taps so everyone was safe!
They first do 5 gentle taps while holding the nail on the line with their fingers. Then they are sure to keep two hands on the hammer as they are nailing it in the pumpkin.
This was in use all morning long and all these nails were done in 1 hour of work time! I think I'll draw another face on the opposite side of the pumpkin to get more use out of it!