"Czy widzisz chlopca" means the same as "do you see a boy"
So I can't say "widzisz chłopiec?"
No, the direct object has to be in the accusative case: chłopca.
I think that a noun that follows the verb widzieć has to be in the accusative declination
Google Translate says, "Widzisz tego chłopca?"
First of all, it doesn't. I just checked. And secondly, what should we do with that information?
That would translate as “Do you see that/this boy?”, not “a/the boy”.