I accidently deleted my grandson's text messages. how do I get them back

You likely don't.

If you use iOS 16 you can tap edit from within messages to see if they may reside in recently deleted.

Otherwise, if you have a device backup that contains the message(s), and you do not use messages in iCloud, you can factory reset and erase your device and restore it using your backup. You'll lose anything new since the backup was made but you might recover your messages.