A sippy cup assembly including a valve insert having a sheath and a stabilizing core is disclosed. The sippy cup assembly generally includes a container, a lid releasably coupled to and enclosing the container, and an annular seal for preventing liquid from leaking out of the container. The valve insert maintains form while permitting repeated removal and insertion, in addition to washing and long-term use, by providing that the stabilizing core has a rigidity greater than that of the sheath formed over the stabilizing core. The valve insert may be installed in the lid to correspond with a spout and air exchange aperture to permit egress of liquid and ingress of air upon application of suction by a user. When no suction is applied to the spout, the valve insert prevents leakage of liquid from the container.
