Oil based stain is the only way to go for sure. And cedar is definitely part of the problem. We purchased and refinished a Rainbow playset for my kid a couple y

Oil based stain is the only way to go for sure. And cedar is definitely part of the problem. We purchased and refinished a Rainbow playset for my kid a couple years ago. We sanded out all the rot and old stain and used a very nice oil based stain. It held up much better but the cedar parts till needed constant maintenance. The redwood posts and beams it came with however looked new after a light sanding and staining and there was never an issue with them.