I can't decide whether どこやったんだ would be better as 'Where are they' or 'What did you do with them'; being as it's 'Where did you do with them', which doesn't map to English well.

Paracite said:

I can't decide whether どこやったんだ would be better as 'Where are they' or 'What did you do with them'; being as it's 'Where did you do with them', which doesn't map to English well.

Hmm... In my opinion, I think in general when you ask information about someone, you ask about their whereabouts before what happened to them.

Like how police officer/detectives ask to a criminal/kidnapper "where did you hide him/her" before asking if he/she's still alive (if I'm not wrong, based on Police TV series).

Just a small thing, you forgot the "ni" between "doko" and "yattanda".

I went with 'Where are' first, but it just didn't seem to fit as well; but then again, it's 3AM. If it was どこに居る or similar, then where would be the only choice.

It seems to be roughly analogous to "what (the hell) did you do with (something)", with the "something" being missing (hence どこ).

A more literal translation would be something along the lines of "where did you do away with (something)", but that's kinda clunky in English.