Ouch. Rough itinerary.
I can't answer your question here, unfortunately. I will say that 18 hours at AMS, even in a lounge, is a hell I've endured three times. The first time was miserable. The second time I found one of those relaxing chairs near the upstairs food court to doze in, and it was still about 90% as miserable. The third time I did the smart thing and forked over the cash for a room at the Schipol Yotel. While I don't know if your niece is travelling for school and/or is on a tight budget, but $115 is a small price to pay for a place to doze, use the restroom, shower, and in general get away from everyone else while still being on the right side of security. It looks like some days/nights are available beginning at $62 too, which is obviously a much more palatable price. Throw in a few bucks for some airport food (which will, as you indicated, easily beat the quality of the lounge offerings), and binge some Netflix or something to pass the time in a nice, air-conditioned bed.