I thought the answer involved making sure the recipent's telephone number was used or that the iPad needed to be tethered to an iPhone in hot spot mode. The phone number was strike one and the tethering option was strike two. I was close, but no cigar. Here's the real answer: Text Message Forwarding from your iPhone.
The Text Message Forwarding feature on an iPhone allows you to compose, send, and receive text messages on your iPad.
To setup Text Message Forwarding:
1. Sign into the same iCloud account on your iPhone and iPad.
2. On your iPhone, go to: Settings > Messages > Text Message Forwarding
3. Slide the toggle switch next to the iPhone you want to use for message forwarding
|Enabling Messages Forwarding feature in iOS 10|
4. On your iPhone, enter the security code that appears on the iPad screen
|Message Forwarding PIN confirmation in iOS 10|
That's it! Once the iPhone and iPad are "paired" for message forwarding, you can send and receive text messages on your iPad with all of your "green bubble" friends.