Install a child-proof bifold door lock on the doors. Follow the specific manufacturer's instructions for installing the lock, which is often positioned near the top of the doors, out of reach of small children.
Install the bifold door knobs, which are called pulls, higher on the doors. This places them out of reach of small children and discourages them from opening the doors. Normally, the knobs are about 36 inches high on the doors. Instead, install the knobs near the top, where only you and other adults, not toddlers and small children, can pull them.
Install an electronic door alarm that alerts you when the bifold doors are opened and sounds until an adult resets it. Create a similar homemade solution and hang a bell on the doors, which will alert you when they are being opened. The bell also might startle a curious child enough to prevent her from venturing through the doors.