I'm not sure this is possible.

Maybe buy a ticket for the same dates, fly outbound, split PNRs, change one PNR. That would set you back at least one of the freshly increased change fees, likely some fare difference too.

As far as I know there is no way to combine PNRs together, so you'd have to start off with a single PNR. But this, by definition, means you and your niece must buy the same itinerary.