Draw 4 or 5 pumpkin-y shapes on a clean shower curtain or plastic table cloth.
Your younger chef might need help with facial expressions, so draw those, too.
Carefully consult with your chef which mouths to draw. Happy? Sad? Scared? Angry? Baffled?
Dips for the eyes and nose, cucumber slices for the mouth, broccoli for the stem, and baby carrots everywhere else.
And if you think your guests might be mildly grossed out, get the older chef to quickly make a big one just before the party starts.
Off you go!