We are doing this study to find out what happens to acetaminophen in the body after it is given to children through the vein. Children's bodies may handle drugs differently than adults. Understanding how long the drug stays in the body and how the drug is changed or metabolized by the body (called pharmacokinetics) is an important step in learning what the best dose of acetaminophen for children should be. We are also interested in learning about the safety of this medication when given to children.