Save a the date etiquette

When I sent our save the dates I addressed the envelopes to the adults ie Mr & Mrs xxx, the actual cards had no names, just mass printed, its this date, details to follow.


I am now worried that I've got the etiquette very wrong.
My H2B's niece is complaining to everyone that he children aren't being invited. They have told her that it is only the save the date & that the children will be included. She is insistent that this is the invite & is offended.
We were always intending to fully include all of the family children & make this clear on the invites. She had a large traditional wedding so I do not believe that she could have mixed this up with the invite, especially as there were no location or time details on the save the dates.
She could have very easily just asked us instead of complaining to everyone else so before I approach her I'd just like to know, did I mess up on the etiquette? Every other family member has rightly, knowing us, assumed that their children would be invited too.