Bouncy Castle PVC and vinyl PVC are flexible plastics that are pretty much wipe-clean, so if you do find a mark you should be able to wipe or wash it off easily, if not do get in touch.
Sterling silver is 92.7% silver and 7.5% other metals, usually copper. This can tarnish with wear and exposure to things like salty air, perfumes, hairspray etc. The good news is, unlike silver plated jewellery, you can revive a tarnished piece of silver. Silver polishing cloths are widely available, and (used gently), these will bring your silver back to it's shiny self again. Store it in a jewellery box to help prevent tarnishing.
T-shirt yarn can be spot-cleaned with a sponge & soapy water. If it needs a deeper clean it can be washed as you would wash a t-shirt. Reshape whilst damp & air dry. It is best stored out of direct sunlight to help prevent fading.