Abstract: A convertible combination shopping cart seat liner and diaper bag for use as both a seat liner and a diaper bag and methods of converting the same are provided. The convertible combination preferably has a flexible container sized and shaped to flexibly position within a shopping cart seat of a shopping cart. The flexible container has a bottom and a plurality of walls connected to and extending upwardly from the bottom. The plurality of walls include a front wall, a rear wall, and at least a pair of side walls connected to and extending between the front and rear walls so as to define an inner chamber for positioning a child and child diaper items therein. Each of the front and rear walls and each of the pair of side walls has an outer surface thereof sized and shaped so as to simultaneously abuttingly contact the inner surface of a corresponding wall of a shopping cart seat when the container is positioned therein.