With younger children, there's the additional concern that they may not really understand what gymnastics fundraisers are all about, and their attention spans are pretty short. Basically, the key to getting everyone on the same page and working in a positive way toward the same goal, is making it fun for them. If you can show them that gymnastics fundraisers can be fun, not only will they do well with their fundraiser, but it will set the tone for all their future fundraising efforts. Choosing a product that they like is pretty important.
Of course you need to keep your supporters in mind, but there's no reason you can't take the kids' opinions into account, too. They at least need to feel like they are being consulted, like what they want matters. After all, you're doing this for them, so they should get some say. Do try to encourage them, however, to lean toward gymnastics fundraisers that will work well in your community.
Without going into too much detail about the money aspect, explain to them how gymnastics fundraisers are directly responsible for providing them with the things they need, and allowing them to travel for things like interstate competitions. If you can get the kids excited, there nothing you can't do with gymnastics fundraisers.
There aren't many people out there who would say no to a cute, outgoing kid asking for help to continue with their sport. It's a pretty easy sell, and that is a great part of the formula that makes gymnastics fundraisers work so well.