How are you making the "neckline" Nicoley? You could make it expandable by putting buttons, snaps, or velcro on the two end pieces hence making it last a lot longer. I'm partial to snaps or velcro myself.
If it's more like a mini shirt (ie. one that covers the shoulders, back, and entire front for the more adventurous eaters...) then putting a slit at one of the shoulders with some buttons or snaps would work out fine.