Be very careful if you decide to try selling door-to-door or otherwise approaching strangers. Never allow your child to do this on her own, she must always be accompanied by you or another trusted adult. Never enter a stranger's house, either. Always make sure that all sales are transacted on the doorstep.
Try to sell the chocolate at your place of work. This works best if you happen to be the boss (subordinates have far more incentive to buy than do coworkers). If you're not the boss, you may need to check with a higher-up to see if such workplace solicitation is permitted. Chocolate bars may actually sell themselves if you place the box out on your desk. Gift wrap, magazine subscriptions and other items might take a bit more work.
Check with local businesses to see if any of them would permit you to put a box of candy out by the cash register. Make sure it is clearly labeled as being a benefit for your school, and make sure the business owner knows this, too.
Try the tried-and-true method of hitting up friends and family members. Don't hit too hard though, since you probably intend to have these people in your life for longer than your child will be attending his current school.
Be prepared to pay for any candy bars that do not sell. If you have requested two boxes of 50 bars each and you only sell 35 bars, you may return the unopened box of 50, but you will need to pay for the 15 bars from the first box that did not sell. Just think of it as a contribution to our child's education, but one you cannot deduct from your income taxes.