1. "an illness" means any type of sickness you have. If you caught a cold then you are "ill". You have an "illness". "Being ill" means being sick. "a disease" is a type of illness that you have for a longer period of time. For example, if you have HIV then it is a disease. It doesn't go away after a few months. It is a critical illness. 2. "children" are very young human beings in general. If you have a baby then it is your "child". It is "one child" - "two children" - "three children" and so on. When the "child" turns 13 then he becomes a "teenager". "kids" is colloquial. It means the same as "children". It is "one kid" - "two kids" - "three kids" and so on. You don't use "kids" in formal (written) speech. "toddlers" are "children" who are not babies anymore but still very young. "A toddler" is one to three years old.

"illness" is a general term for sickness "DISEASES" is a specific type of illness, like if I got a cold, I wouldnt say I got a diseases "TODDLERS" medical term for smaller children, with specific age range "KIDS" more verbal term for children "CHILDREN" general expression for younger age human beings