"When were my parents telling that to you?"
Translation:Quand mes parents te disaient-ils ça ?
"quand" - question word, goes at the beginning.
"mes parents te disaient ça" - statement word order ("my parents were telling that to you").
"disaient" becomes "disaient-ils" as the verb-subject inversion for a formal question; "ils" isn't needed in the statement but when inverted it's added.
Does that help or is there more that's confusing you?