iPads on Business account need to group message
If you are using the same Apple ID to login to all the iPad, how will you identify who sent which message?

If the identity of sender and recipient are irrelevant, and you just need the message to be synced across all the iPads, an inelegant but simple approach is to send the message to yourself, i.e. the same Apple ID which is logged into iPad.

The downsides will be that the each message will appear twice, once for sender and once for receiver. Also, the message will be marked as read immediately.

The solution discussed above is not elegant and it is recommended that you register each iPad with unique Apple ID, and create an iMessage group conversation with all the recipients added.