Sounds like a fun party! If you decide to do a pink pumpkin-- using two bundt pans would be the easiest as you don't have to worry about carving. You could then use black fondant for the stem, vines, and face details. Another idea, and you could do this on a round or a sheetcake, is to have a pink background and then black fondant halloween cutouts for your pattern. Just get the small Halloween shaped cookie cutters, roll out your fondant, and then cut them out and put them on your cake. Ta da!