Hindsight is often 20-20 and, at this time of year, “The Dean" is always deluged with queries from students who have made errors on their newly-submitted applications and want to know how to amend them. In many cases I suggest doing nothing. I point out that highlighting a missing apostrophe or extra pronoun might even do more harm than good.
But there is a major difference between an activity that you have undertaken for 30 hours a week (that's a big chunk of your time) versus just 3. So, here's what I suggest:
1. Look up the name and contact information for the Stanford admission official who oversees applicants from your high school. If you haven't found it already, you should see it here: https://admission.stanford.edu/counselors/officers.html
2. Write a brief email to him or her explaining your application mistake. If you can comfortably add a touch of humor, then go for it. (EXAMPLE: “In reviewing my Stanford application this morning, I noticed that I'd said that I babysat for my little brother 3 hours per week last summer. But I meant to say 30 hours per week. If my mother were to see my application, she might wonder what I did with Stevie for those extra 27 hours, so please make the change for me! Thanks … and apologies."
3. Review your entire Stanford application to be sure that there aren't additional errors that you need to fix. (You don't want to have to send yet ANOTHER correction note!) If there's anything major, you should add it. But if you find some small mistakes (e.g., you said “it's" when you meant “its"), let them go. Perhaps ask a parent, friend, etc. to help with the proofreading. It can be hard to edit one's own work, and the stress of the application process can somehow make it even harder.
Deciding whether or not to correct application errors post-submission can be a tough call. For instance, if your application said, “Whenever I can, I quit," although you REALLY meant, “Whenever I can, I quiLt," then you better send an update pronto … perhaps one that includes a photo of the bedspread you finished last Friday. But if your application said, “Whenver I can, I quilt," then you should certainly leave it alone. (And I bet you even missed the typo in that sentence. And even if YOU didn't, a busy and tired admission official probably would.)
Similarly, don't split hairs over misrepresented hours per week or weeks per year, when the error doesn't alter the big picture. (These numbers can be confusing to compute if your hours are very irregular. The Common App doesn't allow you to say that you take part in some endeavors nearly all weekend with little sleep but then have several months off before a repeat performance. Providing the average hours-per-week rarely tells the true story.) But if you inflated a minor commitment into a major one or–as in your case–you didn't give yourself nearly enough credit for your participation, then sending a correction is important.
Admission officers are typically not as critical as many students fear that they are. Sure, it can work against you if your essay is rife with spelling errors and typos. But most admission folks are reading so fast that the smaller screw-ups may just slip by them. They also understand the pressure that their applicants are under, so don't beat yourself up for making a mistake, but just make certain that you don't need to send more than one corrections message.