"They are having an accident." is also accepted as correct. Which makes more sense if it is actually happening in front of us now. We would expect more information with the other version, for example: They have an accident every other year! How do they have such bad luck? or "They have an accident when they are not paying attention to the road. So we don't want anyone to text while driving."
had an accident" is more common than "they
have an accident." Would "they
are having an accident" as the present-tense translation sound better to you? It would still be uncommon--usually by the time you could comment on an accident, it's over.
Perhaps in the context of a toddler . . . .