Vivarium
 

Why would Aliens need Earthlings to raise their children ?

Why are you sure it was Aliens?

why was they using people to raise that thing, I don't know, its too weird to even contemplate.

Maybe just to give it the most rudimentary skills to pass of as human so that it can fool its next victims

I don't think they were aliens to begin with.In the very first minutes of the movie we saw a couple of cuckoos and the whole movie was comparing the nature of human beings to cuckoos.How the boy ruined his parents life and unintentionally killed them was no different from what happened in the cuckoos' nest.It was a way of showing how nurture and nature,both work.Remember their son's very last word to his mom?"whatever".It was what his mom told him when she couldn't stop him from watching TV when he was a kid.It was like "you reap what you saw".
And it was about how we do what we do because we are told so by society.As if we are programmed to do things in this way.The couple wanted no kids yet they ended up raising one.They didn't like what they ate but it was what they were given and they had no say in it.