A linking device is configured at the bottom of a playpen. Two of the bottom pipes at opposite side pivot to a base seat and further pivot to the linking device, which is engaged by a locking device so that the bottom pipes and the linking device are secured at an unfolding position. To fold the hub assembly, using a strap or by pushing the locking device directly pivots the locking device to disengage from the linking device and the bottom pipes may rotate to a folding position. To restore the hub assembly to the unfolding position, the linking device can be pressed to push an inclined surface of the locking device to impart a force to the locking device to pivot the locking device until the linking device snaps to engage the locking device, so the hub assembly is secured at the unfolding position again.