Rebekah – I don’t understand how it is that your twelve year-old niece could write letters to other residents without your knowledge of that, or of the letter’s content.
Be that as it may, as your niece is a minor her actions on this occasion were your responsibility, and that is why the Strata Manager has sent you a letter about the incident, presumably at the request of those other residents whose children were the recipients of your niece’s letter/s.
It is unusual for a Strata Manager to react to complaints made directly by tenants, and whilst you’re only assuming that to be the case, if you’re in NSW then the document you’ve received has no consequences as it’s merely a letter (of complaint) as opposed to a Notice-to-Comply with a specific By-Law issued in THIS format under the provisions of the NSW Strata Schemes Management Act after a Meeting of the Executive Committee so determines.
So if you’re greatly offended by the Strata Manager’s letter, then by all means respond, but personally as that letter carries no consequences to you or to your niece, I’d just tell her to in future act more responsibly (instead of as the neighbours do), and then suck it up and move on.