Security

Click on this link "If you think your Apple ID has been compromised" --> https://support.apple.com/HT204145

'Checklist: If you want to see if anyone else has access to your device or accounts' click here --> https://support.apple.com/HT212021

Security and your Apple ID - https://support.apple.com/HT201303

Contact Apple for help with Apple ID account security - https://support.apple.com/HT204169 "This article provides country-specific Apple Support contact information for customers seeking help with their Apple ID password or other security-related issues." Better for directly reporting an issue.

Change the passcode: Click on this link to learn how to 'Use a passcode with your iPhone, iPad, or iPod touch' --> https://support.apple.com/HT204060

I can't say for sure but I doubt they can see what is happening in What'sApp on your device, but then I have never used WhatsApp. They would probably have to be logged into your WhatsApp account and I don't know if it is possible to do that twice from different devices.

You should still change your password and PIN if you know somebody else does know them.