My iPhone was stolen and I can't pair the new one with Apple Watch

For your watch→How to pair your Apple Watch with a new iPhone - Apple Support

For your iPad, you were never able to pair an iPad and an iPhone. If you log in to the same Apple ID on both devices they will share content through iCloud, and other features through continuity. And if you add your email accounts to both devices your contacts, calendars, notes and reminders will appear on both.

If you never remove the stolen phone from your Apple ID it will be useless to whoever has it. But beware of a common phishing scam: You may get a text or email from “Apple” or “Apple Security” (or sometimes “Police”) saying your phone has been found and to click on a link to see its location. This is NEVER from Apple or from law enforcement; it is from the thief or fence to steal your Apple ID and identity. Delete any such message without responding to it. Apple never gets involved in lost or stolen devices→Avoid phishing emails, fake ‘virus‘ alerts, phony support calls, and other scams

Contact Apple Support:

"My 12+ was stolen it is now in MX I can’t get my iWatch or iPad to pair with new phone"

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For reference...

Securing your Account:

Having been stolen, someone may have your credentials. So, do four Things:

A. Re-Associate All Devices:

Disassociate all devices from your Apple Account, first.  Sign out of iCloud on all of your devices.. Go here to check the devices associated to your account: Check your Apple ID Device list to See where you're Signed in - Apple Support. Then, log in to your Apple iCloud Account on each device.

B. Verify Your Devices:

Verify that your the devices logged in to your account or officially yours, by using this link: Check your Apple ID Device List to See where you're Signed in - Apple Support

C. Consider Modifying your Credentials:

By all means, consider modifying your credentials, starting with a new password.

• Modify: your login credentials (Using Both a New eMail and a New Password for each and all sites)
• Associate: a different method of payment to your account
• Enable: Two-Factor Authentication for Apple ID and for all other accounts that have such an applicable option.

D. Change the Password and Passcode:

• Mac: Change or Reset the Password of a macOS User Account in macOS Mojave or Earlier - Apple Support
• iPhone: Change your Passcode or Passcode Settings - Apple Support

Contact Apple Support:

"My 12+ was stolen it is now in MX I can’t get my iWatch or iPad to pair with new phone"

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Troubleshooting your Account Devices:

Having issues like this with your account devices, contact Apple Support. Ask them if they can fix this information to how it should be: Though use of the new iPhone. They’ll see what’s going on and will run you through the fix.

Apple Contact Info:

• Contact Us - Choose Locations
• Proceed from there, as necessary
• Why the Wait: Phone calls are taking a bit to go through at the moment, due to lots of calls. With the Coronavirus Pandemic, many are at home at the moment, with much time on their hands to contact Apple. So, just stay on the line, and you will get through :)