Not all onesies are pajamas, but certain onesies are pajamas. Adult onesies are usually always sleepwear, yes. However, onesies for kids can be either regular daytime wear or pajamas, depending on what the material the onesie is made of. For example, if the onesie is of a fleece, or sometimes terry cloth variety, it is a pajama onesie made specifically for nighttime wear. This is because these kinds of fabric are very soft and warm, as they retain heat and moisture. Normal, everyday onesies for kids are typically made with a cotton, polyester, bamboo, or other mixes of fabrics.