From what I have gathered, it is pretty normal in America/English to call 16 years old "children" (although Guardian is from the UK), in contrast to Germany/German, where apparently they normally wouldn't be called "Kinder" but "Jugendliche". So maybe it is just a linguistic or cultural difference to your country.
Edit: I think it is also a method to differentiate K-12 schools from higher education. Although "college kids" does also appear to be used often in America (don't know about the UK). I am not sure whether "kid" and "child" can refer to different age groups.