Who first thought of the idea that there are aliens from outer space? My eight-year-old 
asked me, and I don’t know the answer.
Since we’ve known that other planets were, actually, planets, it’s been a common idea.
Some would say Angels were the first ‘aliens’, or even humans were (We’re from Eden, not of this earth), at least in christomythology. This idea persists throughout much of history. It would be impossible to credit the idea to an individual, and if some conspiracy theorists are correct, the Ancient Sumarians had their own idea of aliens, although I don’t know the probability of that being their actual belief system.
There a whole section of a wikipedia article about this.
The short version is the greeks were first to come up with a system that differentiated between aliens and demons/gods/angels/whatever.
Modern beliefs in and narratives about extraterrestrials have a lot of features in common with earlier fairy beliefs in the English-speaking world, down to an outer-space origin (not that “outer space” was a term used in the 19th century; I believe the expression was “from the skies”). In other words, contemporary alien beliefs are just a contemporary modification of a rather longstanding cultural belief. I’ll refer you to Narváez’s The Good People for details.
The notion that there’s an inhabited world beyond this one goes back as far as recorded mythology. That there was some sort of Axis Mundi connecting us to an upper world. Originally those worlds seem to have been reflections of earth, but the idea that (1) there was somewhere else with life and (2) that life visited us occasionally, those are ancient ideas.
That’s a really good question. Buy your kid something nice.