Stop sharing from iPhone to iPad

You can prevent Message Forwarding from your iPhone to your iPad - but this won’t inhibit access to messages already received by the iPad - or synchronised via iCloud.

On your iPhone:

Settings > Messages > Text Message Forwarding - explicitly disable the iPad from the device list

Similarly, you’ll not be able to prevent access to your email without access to the iPad - or by changing your email account password(s).

If you are unable to retrieve the iPad and properly prepare the iPad for a new user, if your are concerned for your privacy - and to prevent access to sensitive information or data - you might be best advised to remotely wipe the iPad using Find My.

Open the Find My App on whichever device you have to hand - then select the Devices tab. From the list, identify the iPad that you wish to wipe - and Tap. This will open the management page - where you can select Erase This Device.

Sounds like you didn’t properly prepare it for someone else to use.

This is what you need to do: What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

You are very welcome.