As long as the funds don't get diverted away from the 5th graders, and the parents and school agree, it shouldn't be a problem. What have you thought about doing for a fundraiser?
Every year, our 5th graders have a big, expensive day camp.
So, the year before the 4th graders and their parents are pushed to raise money so every child can go. This puts a lot of pressure on a small group of people.
What are you opinions of having all proceeds raised go to the PTO (and not the 5th grade). Then at the end of the year the PTO can make a contribution to the 5th grade camp.