because of "idę", most often if the verb indicates movement the case changes, https://en.wikibooks.org/wiki/Polish/Prepositions_as_hints_to_declensions
"I am going with my mom to the station" is as perfect and as acceptable a translation in English. There is/was no need to mark it as wrong. There is however, a general need for better English explanations of grammar and idiom. Perhaps you should have a native speaker check or at least examine your texts and corrections before putting them out there?