A regular (non-waterproof) cast becomes firm to the touch within 10-15 minutes after it is put on, but for the first two hours it is soft and can easily be dented or cracked. If your child has a walking cast (weight-bearing cast), they should not walk on it for two hours after it is applied.
Do not place the cast under water and protect it from splashing. Cover the cast with a plastic bag and tape the opening shut. Even with a cast covered, do not allow your child to place the covered cast in the water. You can also buy special plastic covers at some specialty stores but they can leak when placed under water. It may be easier for your child to take sponge baths while the cast is on.
If the cast becomes soiled, clean it with a slightly damp washcloth. Your child should not put clothing over the cast until it dries. Protect the cast by covering it when your child eats or drinks. If a cast gets wet, immediately dry it with a blow dryer on the cold or cool setting. Children can be burned with a blow dryer on the warm or hot setting.