S.G.
You say "Sorry, I have my own kids fundraisers." or "No thank you." or "I don't need any X right now." It is not rude of them to ask, and it is not rude for you not to support. When my kids have a fundraiser I will send out a mass email. Anyone who receives the email is free to support or not. There are times my friends kids are selling something that I want, but my own kids aren't selling and vice versa. In fact, I think I probably supported more of my friends kids fundraisers last year than I did my own kids, simply because they had better stuff. Many parents don't let their kids sell door-to-door anymore, so they instead hit up friends and family.