In everyday speech when answering question like "Do you think they have kids? Or not?" you'd say "tak - mają dzieci" (emphasis on having). "Oni mają dzieci" sounds a bit like a starting to say "therefore they cannot come to the party (emphasis on them being affected). Pronouns are commonly dropped - since form of the verb is sufficient. If included they may shift the emphasis.
Oni is for masculine or mixed (where at least one person is a male), and one is for feminine (where all members of a group are females).
If you're translating from Polish, it's simply they because English doesn't distinguish between the two. If you're translating from English, you need context, but on Duo, it can be either because there's no context (unless there's an audio recording where you can hear whether it's oni or one).