Lock individual apps

What is your intent on doing this? You can't lock them as such but there may be doing what you want to do if we knew what that was.

Use Restrictions on your iPhone, iPad, and iPod touch = About Restrictions (parental controls) on iPhone, iPad, and iPod touch - https://support.apple.com/HT201304

No, iPhone is not a share device.

You can lock the whole iPhone up with a passcode, no need to individual lock them.

If you want your kid to use a particular app, use Guided Access

Read here: Use Guided Access with iPhone, iPad, and iPod touch - Apple Support