A control unit is slidably mounted on a top corner of a playard and connected to the slider inside the seat through a linking unit passing through a standing post of the playard. When the control unit pulls the slider to move relative to the seat along a first direction, a braking pin slides relative to the seat along a second direction and disengages from the hub to release a wheel. When the control unit is pushed to lock the wheel, the bias force from springs moves the slider and the braking pin to slide along the opposite direction, leading the braking pin to engage with the hub to brake the wheel. The wheel can be operated to brake or release by a remote control unit.