How do I lock my photos??

You put a Lock screen passcode on your device (with Touch ID or Face ID if you wish), and you don't let anyone know what it is. Then no one but you can get into your device to see what you have on it.

Best,

GB

You can only lock your iPhone, and if want a child to use use then

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

Don't allow anyone to have access to your iPhone.