From the sound of things, going to agree with both above. Can't password protect a folder of apps, Guided Access might work, and possibly turning on Restrictions, depending on what trying to lock down.
Depending on the app, some do have ability to turn on PIN/passcodes to access (eg. Dropbox, other local file management apps, Notes).
Or: don't let others use your device, pick a better device PIN/passcode, reboot phone or put into Distress Mode before going to bed (if concerned about SO trying to open phone with fingerprint while you are asleep).