If the arm is injured, your child should wear the sling provided by your physician to keep the injured part elevated above the heart and limit swelling. Occasional wiggling of the fingers or toes will also prevent some swelling. Give acetaminophen or ibuprofen in the appropriate dosage for pain. Continue this for at least the first 48 hours. Don't get the cast wet.
Wet plaster can become soft and crumble.
Wet padding under a fiberglass cast can cause skin rashes. If the cast becomes wet, dry it with a hair dryer. To avoid getting the cast wet, use a washcloth and wash basin to bathe. Enclosing the cast in a plastic bag for a bath or shower usually doesn't work. Always cover the cast with a plastic wrapping to protect it when you are going to be around water, even when it is raining.