Is the Romanian asking where they were just before they disappeared, or where they went when they disappeared? The latter would be (at least in the Southern USA), "Where did the boys disappear to?
Your version isn't an English regionalism, both your and Butterfly's versions are correct in both countries, just the tenses are different.
☺ So, just to clarify, if Duolingo could add the little word "to" at the end of the English translation we would all be happier. Moreover, if both "had" and "have" could be accepted as alternatives we would all be delighted.
I don't like ending sentences with prepositions and I think you meant "did" instead of "had"? I'm only here to be difficult. :)