It sounds like you guys are in a really good place that many would be jealous of
If everything is working fine then why would you even bring up the word?
Is there something more you want from them that requires you to identify with a specific label? If it is merely the word polyamory that disturbs them, and there will be no noticeable difference in your relationship regardless of how it is referred to, than can't you simply let go of the word?
If you are looking for something more closed like a polyfidelitous agreement than perhaps more talk would be necessary. Otherwise enjoy yourself!