Use the built-in DISK UTILITY, and make the folder a Password protected Disk Image.
Applications folder>>>Utilities Folder>>>DISK UTILITY.
Disc Utility>>>File>>>>New>>>>Disk Image from Folder>>> Find the folder you want to "lock", you'll need to choose the "128" encryption then enter the Password you want to use.
This turns the folder into a .dmg... In order to open it,to gain access to your folder, you/they will need the password.