What to do if somebody took my iPad mini

Start here:

Locate a device in Find My on iPad - Apple Support

Here is Apple’s advice for lost or stolen devices:

If your iPhone, iPad or iPod touch is lost or stolen – Apple Support

Use the Find My app to locate a lost or stolen device – Apple Support

If the Find My iPad feature was enabled prior to it being “misplaced”, then if active, you would stand a good chance of locating it. 

However, if the feature was not activated prior to being misplaced - or the iPad has been inactive for more than 24 hours, the last known location will not be available - and the opportunity to use technical means to recover the iPad have been lost.

An iPad that has been instructed to erase using Apple’s Find My service will only erase when it receives the command to do so - this, logically, being determined by the iPad having an active network connection. If the Find My service indicates that the erase is pending, then the erase command has not [yet] been successfully delivered to the iPad; the status will change when [if] the command is successfully delivered to the device.

Nobody can gain access to your iPad without successfully entering the correct device Passcode - and for this, there are a limited number of tries before the iPad will automatically become disabled. As such a brute-force attempt to access the iPad, by guessing the correct passcode, is highly unlikely to be successful. Only if your device Passcode is known to whoever finds your iPad will any locally stored data be at risk of discovery.

iOS/iPadOS is architecturally designed to protect the owners data. 

All locally stored data is encrypted; by design, the only copy of the encryption keys necessary to access local data are held within the Secure Enclave - the device security chip. The device Passcode unlocks the Secure Enclave, which in turn releases the encryption keys to the Operating System while the device remains unlocked.

If an incorrect Passcode is repeatedly entered, the Secure Enclave automatically wiped and the device disabled - an operation that erases all stored encryption keys. This is known as a crypto-erase. Once erased, all locally stored data is permanently beyond reach and cannot be recovered.

Beware of a common phishing scam: You may get a text or email from “Apple” or “Apple Security” (or sometimes “Police”) saying your iPad 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

uimsg wrote:

Is there a way for me to report the serial number of the lost iPad to prevent it to be resold as a refurbished iPad? Along the same question, could the thief sell it as a refurbished iPad without unlocking it?

No, you can't prevent the iPad from being resold. People sell iPads on eBay, Facebook, by word of mouth. And, yes, the thief could claim anything they want when they resell it. They have already proved their dishonesty. But, it's still going to be activation locked. That means the unfortunate person who buys it will be out of money. And, as long as you had a passcode on it, your data is encrypted and inaccessible.

1) There are no other means to trigger a remote erase. If the iPad does not have an active WiFi or Cellular network connection, it cannot communicate with Apple's servers. There is no other mechanism with which to remotely manage the iPad.

2) While locked with your device Passcode, the iPad will display nothing other than your Lock Screen - or, if so configured to be accessed from the Lock Screen, the Notification Centre.

3) Consider changing your AppleID Password. If you are concerned, change the account passwords for sensitive accounts (e.g., banking and financial) that are stored in your Saved Passwords (i.e., your keychain).

You can report loss or theft to the relevant authorities should you choose to do so, but this will have no impact upon what will happen to your iPad in the hands of someone other than you.

As Find My was enabled prior to loss, your iPad will be Activation Locked if a reset is attempted; with an active Activation Lock, the iPad is uselss to anyone - and Apple will offer no assistance whatsoever to anyone other than the bona-fide owner.

Many stolen devices are offered for sale to the unwary. If purchased, the iPad will still be useless to anyone other than you.

In summary, unless your Passcode is known to whoever has your iPad, your data is safe. Only a limited number of attempts are possible before the iPad will automatically wipe the Secure Encalve (i.e., the iPad's security chip) and disable the iPad - at which point all locally stored data is permanently beyond reach. All locally stored data is encrypted; without the encryption keys stored within the Secure Enclave, no data can be recovered by anyone. Once disabled, the Activation Lock remains active - rendering the device useless.