With a little guidance and a couple of days, they will do the job for you.
1/8" or larger drill bit
smooth peanut butter
Step 1: Drill Guide Holes
Start with an uncut pumpkin. You do not need to open it up and scoop out the guts. The squirrels do that for you.
Using a drill on low speed to reduce spattering, start out by drilling holes where you want the final holes in the pumpkin to appear. As shown in the first picture, I have drilled 1 hole each for the eyes, nose and a line of holes for the mouth. Drill all the way through the pumpkin flesh.
Drill around these holes to make the openings bigger as shown in the 2nd and 3rd photos.
Step 2: Fill the Holes
I have tried using a cake decorating syringe but it did not work particularly well as the peanut butter is too thick. And it ended up being more work to clean than to have carved the pumpkin in the first place.
Once all of the holes are filled, wipe the surface peanut butter off all of the places you don't want the squirrels to eat. Its OK to leave surface traces of PB on the areas where you want the holes.
Step 3: Place Pumpkin
You can add a little more peanut butter to the pumpkin to direct the attention of the squirrels to areas you would like "carved" quicker.
The squirrels will eventually break through the flesh into the pumpkin and take away all of the seeds.