The present invention relates generally to the field of child containment devices, and specifically to a child containment device including a mechanism for holding a mattress in place on the floor of the containment device. According to an example embodiment, the invention comprises a side wall enclosure formed from one or more sidewall panels and a floor panel coupled to the sidewall enclosure to define a receptacle for receiving an infant or child. The invention further includes a mattress for placement upon the floor panel. A mattress retaining mechanism is coupled to the sidewall enclosure and is configured to hold the mattress in place against the floor panel, such that horizontal and vertical movement of mattress is considerably reduced or eliminated.