To make your own devices share,make sure everything is on the same Apple ID and password. Devices sync perfectly. For messaging, make sure messaging is enabled to make/receive messages.. .How to Enable Messages in iCloud on iPhone or iPad
You should also go to Settings > Messages on your iPhone and select your other devices under Text Message Forwarding. This will insure that non-iMessage texts (SMS) will also sync to your iPads.
Only devices using the same iCloud account will show up.