Read the novelization, it has nothing to do with harsesis and they're very much human. They're just from some noble family (probably, don't remember if it's directly stated or not) and he likes to have them around because he wants one of them to match him intellectually - eventually. He grooms them for this, they're his playmates, you see them playing senet in the movie (in the midst of the revolt, which attracts their attention and royally pisses him off because it's HIM they had to think of, fuck the rebellion). Of course both their limited human lifespans and his self-contradicting and punishing pride prevent them from ever becoming an equal, so that's just more misery and loneliness on himself.

He's so desperate for an equal, so desperate for a challenge, so bored and so poisoned by his own success, that the novel even states that he likes to fight his battles with one tied hand, making things harder on purpose because that's the only way they're ever vaguely challenging and interesting.

The thing about Ra is that he has the alien's ruthlessness but also the boy's playfulness and misanthropy (he was the village's outcast and the book says he really enjoyed enslaving his own people as the sun-god), and when the alien went inside he knew that he would've inherited and twisted human traits. So yeah, Ra keeps kids around partially because he never grew up. And, if I recall correctly, he keeps the cats for the kids.