An optionally deployable side seat for a baby stroller, to seat a second child, is secured by hand-operated connectors to nearly any baby or child stroller, without tools. The side seat faces to the side of the stroller and has a deployed position in which two wheels mounted in tandem on the side seat frame contact the pavement. For stowing the side seat remains on the stroller, and two quick-release lockable joints, in the position of arms on the side seat, are released and raised upwardly, which folds the seat bottom up against the seat back and raises up a lower frame portion that supports the wheels.