I think one major point of these 'where do you see yourself in the future' essays is get a sense of your personality--don't write strictly about career/job stuff. Throw in a sentence or two about a hobby you plan to pursue (I remember writing that I hope to someday grow a garden). If you think you'll have kids in 10 years, I don't think it's unreasonable to mention that too...just be strategic with what you say and come try to sound like the multi-faceted human being you are, not a medical robot.