Why do I have to unlock an iPad to charge it? When I plug in the "iPad (6th generation) Wi-Fi + Cellular" to the MacBook, it always says "unlock this iPad to use accessories."  What it doesn't say is that it also won't charge until I do so.  Took me several times to figure this out.  (Has a cover, so I didn't see the message.)

But the iPhone 7+ doesn't do that.  Both have IOS 12.4, so there must be a setting that I am not recognizing.  Or are there distinct unlabeled "flavors" of IOS?

How can I make it start charging as soon as plugged in, even if screen is locked?
The 'unlock to charge' is a new security feature since iOS 11.4. If you charge from a device that asks to communicate over USB you have to opt in. 
Alternatively, just plug in to a 'dumb' charger or employ a USB data block dongle / condom to physically prevent “juice jacking”. If there's no potential data connection, there's no restriction on charging or prompt to unlock.

To elaborate a bit, this design prevents hardware USB devices read data from your device. Remember when the FBI wanted to get into some guy's phone &amp; tried to force Apple through the courts to 'let them in'? This is the current state of play in the hacker 'war'.

It can be disabled if you don't think you have any data on there worthy of government or black hat hacker intrusion.

Apple KB - Using USB accessories with iOS 11.4.1 and later

Settings > Face [Touch] ID &amp; Passcode > USB Accessories