Double coding is used to create a subtext or second reading of a text that speaks to a multiple audiences or sensabilites. This is very common in the use of adult humor or subtext intended for the older audience of a childrens show. It is also used to target multiple demographics of a show, while not ostracizing the main market.
Because of the sensitivity of what is considered appropriate on children’s shows double coding can also be used to establish aspects of a character that if stated explicitly would not make it past the censors. "Questionable" aspects of a character will be consistently, and carefully coded over the course of seasons.
In Hey Arnold! Helga's mother Miriam was coded as an alcoholic. She suffers from depression, she is disoriented and irresponsable (will forget to pack her child's lunch). She is found "asleep" i.e. passed out in different places in the home. She is often seen with a smoothie in hand (or making one), clutching at her beverage as she slurs her words, and saying "I need a smoothie," when things get rough.