When we first meet Owen, he is sitting on the jungle gym eating them and singing the jingle to himself. (Although I was a teen in 1983, and in fact growing up in the same media market as Matt Reeves, I must admit that jingle doesn't ring any bells with me. I guess I may be a bit culturally illiterate about my own childhood. ) Also he is chewing on Now & Later candies. When his mom calls him, he takes the empty wrappers out of his coat pocket and hides them clumsily under the snow. Now & Later is something that he will hide from his mom.
It is Owen's purchase of Now & Laters that lead him also get his little knife - the symbol of his rage and his desire for revenge. Owen intends that this knife be destined for human flesh. When he and The Father exchange eye contact, Owen is feeding himself candies from the point of the knife, and in fact the knife is in his mouth. Subsequently we see that The Father also has a knife that is destined for human flesh. However, from the point of The Father's knife it is Abby who will be fed - or at least she would have been had he not tripped and spilled the jug. (Was it Ricky the poodle who was driving the snow plow? )
After that, Now & Later candies are featured in the "I could try one" scene. Owen attempts to share something of his life with Abby, and it is a part that he will not share with his mom. While Abby wants to share Owen's favorite thing, there are just some parts of Owen's life that can never be part of Abby's. Now & Later.
And then of course, there is Abby's note.
I must be gone and live,
or stay and die.
Then Owen turns it over and on the other side it says, "Now & Later".
By the last time we see Owen, the film has lead us to understand that Owen will pick up where The Father left off. Owen is now the thing that will extend Abby's existence forward in time. He is once again singing the jingle, "Eat some now, save some for later." This resonates with Abby's existence. It is indeed literally how she deals with the people who come into her life. The jingle is describing for us Abby's life in a morbidly humorous fashion. In fact, Abby herself hears the jingle on the radio, after the news report about The Father being captured and taken to the hospital. Even the radio is exhorting her to "Eat some now, save some for later."
I don't know if this little bit of black humor associated with this motif was intended from the beginning by Matt or if it just took on a life of its own. Either way, I like it.