"We go to the boy."
Translation:My chodzimy do chłopca.
Iść/chodzić is a so-called verb of motion, with iść being directional and chodzić non-directional. The Polish National Corpus gives dozens of results for "iść ku" and none for "chodzić ku", which strongly suggests that "ku" is a purley directional preposition and is thus incompatible with the verb "chodzić". You might argue that "do" is also directional, but while "ku" definitely suggest the meaning "towards sth.", "do" can be also just "to sth." (up to a point/place) and is therefore much more versatile.
Apart from that, ku became archaic in most contexts and was replaced by do. I wouldn't recommend using ku nowadays unless it's part of a fixed expression, or if you're writing poems.
Well, at least two people as it's "we", but they are not 'walking over' right now, they go to the boy regularly, for example once a week.
The problem is that this course introduces iść/chodzić very early (kinda logical, because translating "to go" seems like a very basic thing), but they are quite complicated, so it would be a lot better if we were able to provide more context in the sentence.