Translation:They will buy new clothes for their son.
'One'? The pronoun and the noun are both feminine, shouldn't it be a plural 'synów' or a singular 'ona kupi'?
No, not really - Duolingo offers no context, so they could be a lesbian couple rising a son, for all we know. ;-)
also, in Polish it is more possible to interpret this sentence as They will buy new clothes for their (some other people's) son.
And ich is for both one and oni.
Oh yeah, I forgot that 'ich' can refer to theirs as in other people's son or their own in Polish.
I guess it also makes sense in English, although you would typically say 'They will buy new clothes for their own son.' to specify that it's theirs, otherwise it's the same.
And you would say "One kupią nowe ubrania dla swojego syna" in Polish if you meant their own son.
And it is AFAIK more obvious thing to say than in English. Lack of "swojego" makes me always think it is not their own.