"What do their bears eat?"
Translation:Kion iliaj ursoj manĝas?
To make it more transparent, you can say iliaj ursoj mangxas pomon --> iliaj ursoj mangxas kion?. So when the "what" is done something to, you use the accusative form. That's super logical. Although it wouldn't be if that sentence was passive. Is there a passive in Esperanto? Oh yeah, there's one: you use esti+passive participle in the desired tense of the verb, so Iliaj ursoj mangxas pomon. --> Pomo estas mangxata de iliaj ursoj. An apple is eaten by their bears. La supo estis mangxita. The soup had been eaten. La fruktoj estos mangxotaj The fruit will have to be eaten (will be "doomed" to be eaten).
It is to do with the case. In this example, the statement would be 'iliaj ursoj manĝas (ion)'. The bears are the ebject and the thing that they are eating is the object (accusative), so it has an 'n' on the end. We use 'kion' here because the 'what' is in the place of the thing being eaten. If the question is about the subject, such as 'what it is?' we can say 'kio ĝi estas?' without an 'n'. I hope that makes sense...