KNOW ABOUT WOOD
Unlike steel, wood is a natural, organic material and each piece will be unique, bearing its own knots, cracks and variations in colour and, unlike steel, wood will react significantly to changes in atmospheric conditions.
Wood will expand and contract as fluctuations of temperature and humidity affect the water content of the cell structure. In warm sunny weather it will shrink and in wet weather it will expand. As these physical changes do not necessarily take place uniformly throughout a piece of wood, forces are set up which cause cracks and splits to arise. This is a natural process and does not affect the structural integrity of the wood. Please use the following link to see what ROSPA has written about commercial play equipment. https://www.rospa.com/play-safety/advice/wooden-play-equipment.aspx
It is not necessary to stain your wooden frame. It is made from selected wood which has been pressure treated.
This is a process by which preservatives are forced under pressure into the fabric of the timber to protect it from decay and insect damage. If you do wish to stain your wooden frame for aesthetic reasons, please use a water-based stain, which is available from any good DIY store.