Lots of experience. First off .. you don't need to buy tickets ahead of time .. buy a Museum Pass (NOT a "Paris Pass" or "Paris Viste" pass) .
Your niece will walk in with you, the only place I recall having to line up to get my teen a ticket ( even though I had the pass ) was the Invalids Army Museum and theres never any huge line there anyways ( been there multiple times) .
Make sure she brings id to prove age.
The one place you should buy tickets in advance and no pass is useful is the Eiffel Tower. Go on the website, exactly 90 days in advance and purchase tickets for a pre timed slot.. They are non transferable. so the time you pick is "it". They sell out FAST.. some people make a point of being sure to log on at midnight the day their ticket date becomes available.
The Museum Pass can also be used for Versailles( but does not cover the weekend fountain shows.. don't bother I think they are over hyped and cost an extra 8 euros, and the crowds are huge)
The Museum pass will cover your admittance to the Towers of Notre Dame. but it one of the few places you can never use it for line skipping. The line there is LONG AND SLOW so get there first thing before they open in am. ( line moves slowly as the stairs up the Tower are narrow .. and space limited at top, only about 20 people can go up at once.