Pumpkins are in the plant kingdom. They are an angiosperm, which is a plant that has flowers. They are in the Cucurbitaceae family, the gourd family.
Gourds have a thick skin and a soft inside with lots of seeds.
You know what it looks like inside a pumpkin when you cut the top off the pumpkin and pull out the insides with all the seeds and stringy stuff. The other plants in this family are cucumbers, melons, and squash. I couldn't find the class and order for pumpkins. Pumpkins are in the genus cucurbita, but there are many different species. Pumpkins are a vine.