"My mother and my father went to eat soup."
Translation:Mi madre y mi padre fueron a comer sopa.
I hope non-native speakers will not be encouraged to think that the use of "went to eat" is a natural equivalent of a "future in the past" ."Went to eat" would only be said if it expressed a special intention "They went to the restaurant to eat its famous soup." I am not a professional linguist, but someone who has been hearing and listening to English for 75 years since birth. The future formed by go + infinitive" is always the continuous form "I am going to...." .We don't say "I go to..." as a future. "I go to church" means "I regularly attend worship." "I am going to church" means I am on my way there or about to set off. "They were going to eat soup, but he had a heart attack on the way" If this is not a case of another way of expressing future in the past, I wonder if the meaning of the strange English would be conveyed better by "fueron para comer sopa".
it's difficult to tell out of context, but in this case is "went to eat" simply meant to imply travel... for example they specifically went to a particular restaurant to eat the soup? I'm guessing they've used this expression to reinforce the lesson from the previous section that IR and SER have the same conjugation in the preterite.
DL isn't smart enough to make that distinction - it's looking for a particular phrase (mother and father), and only extends that to recognise other valid phrases (like parents) if it's updated. If you think it's a valid translation you need to tell them.
Personally I think you're on shaky ground... You're right, Mother and Father is broadly equivalent to "parents", but if the English sentence uses the words Mother and Father, IMO the sensible thing would be to translate this to "madre y padre".