Kids Captain America Onesie. Avengers Assemble! This awesome Captain America Onesie will have your Marvel manic feeling like they are one of the team! Crafted in fuzzy polyester, the onesie is reminiscent of Captain America's iconic uniform in red, white, and blue. Complete with a lined hood featuring a Marvel badge on the front, this all-in-one will keep your super hero comfy and cosy even when saving the world!