There's no official WhatsApp app for iPad.
You can connect to WhatsApp
through a web browser on your iPhone.
First, make sure you have a WhatsApp account set up.
Open a web browser on your iPad
and go to web.whatsapp.com,
and you'll see a barcode on the screen.
On your iPhone, open WhatsApp
and tap on "Settings" on the bottom right.
Tap on "Linked Devices" and then "Link a Device."
Point your phone camera at the barcode on your iPad screen.
Once you've scanned the code,
you'll see that your WhatsApp is now connected to your iPad.
You'll now be able to view
and respond to WhatsApp messages on your iPad.
When you want to log out,
go to settings on your phone WhatsApp
and then "Linked Devices."
Click on the linked device,
tap "Log Out," and then again to confirm.