Our office handled a similar case in a way I haven't seen mentioned yet. The director of engineering -- that is, the direct or indirect manager of everybody else in our office, but still a team member and not a high-level executive -- brought in an order form for Girl Scout cookies for one of his children, with the following message: I'm going to buy some of these for the office so please vote on types, and if anybody wants to order some for yourself, fill out the form. By doing this he led with his wallet (so to speak), did something for the team, gave everyone a chance for input without spending money, and gave everyone a chance to place additional orders. He bought a generous number of boxes for the team, and people still bought a lot privately. (It looked like it was about 50-50 in the end.)
If your kid's candy bars come in different varieties, you could do something similar. Before you bring anything in, tell your team that you'll be getting some fundraiser candy for the group, they get to pick the types, and if anybody wants to order additional candy, they can do so. Then bring it in, make sure people know where the "public" candy is, and set up a separate box for purchases.