IMessage Access from 2nd Device

Yesterday, I visited my son and ex partner. They have the old ipad with my apple id as the main account. I changed my apple password on that ipad in order to try and download something. I think then, to a degree, my iphone and old ipad were linked again. Today I got a message in a grey box saying that imessage was linked (i think, had just woke up!). That’s not what I want so I changed apple password and removed the ipad as a device from my iphone. My questions are: if someone tries to access messages on the ipad, would they need my new password to view any? Also, would messages that weren’t on the ipad now be on there because of the temporary ‘link’ of devices? TIA