It seems like your coworkers are constantly trying to sell things on behalf of their kids, doesn't it? It's always for a trip to Washington, D.C., for a bus ride to the soccer playoffs, for the entry fee to the robot-building club national championships. If you want to avoid all these purchases without sounding horrible, say this: "Of course I would love to help your kid's cause, here's my phone number. Have them call me this week and tell me more about their trip (project, etc) and I'll buy something from them." Kids are always too terrified to call an adult and explain things, so you can get away purchase-free, without saying no.