Wow, this is a common occurance for us. Our party of 5 (me, DH, DD6, my mom and dad) all go out to eat quite frequently at our local restaurants and we almost ALWAYS get put at a table of 4. And considering there is only 1 child, it is VERY uncomfortable. You mean this isn't normal???
I don't know about parties of 5 at Disney though. Maybe they would try to accommodate you at a larger table.