I understand that "cacheton(a)" is used as a singular noun or adjective in Spanish. You can't use "chubby cheeks" that way, e.g. "She's a chubby cheeks" (Ella es cachetona) or "He is so chubby cheeks!" (¡Que cachetón!). You could use it as a nickname (Hi, Chubby Cheeks!) or say it's something someone has ("She's got such chubby cheeks"), but not something you are. I hope that's clear.
They're in the definition.. ella es cachetona, que cacheton,