Certified GKH Gardening Expert
It depends on the type of pre-emergent and how long that pre-emergent stays active in the soil. If it is a long lasting pre-emergent, then yes, it will stop the pumpkin from growing. You may want to start some seeds in paper cups and then once they sprout, transfer the cup and all into the treated ground. This will get around the pre-emergent as it will not affect them after they have sprouted.