Seine river cruise

There are several big companies that do these and smaller ones as well. One of the biggies is right by the Eiffel Tower (Bateaux Parisiens). They all do pretty much the same route, up river past Ile de la Cite then turn around and back down river in the opposite channel to the Eiffel Tower and turn around to go to their own dock. The sparkles take place after sunset on the hour for about a minute.

1. Yes the three tours I know of all cover the same territory. One end is the Eiffel Tower. Unfortunately, you may not be close to the tower when it sparkles - but you will see it and it will be at least lit up.
2. I did research and it seemed like people recommended Vadettes du Pont Neuf. I've only ridden those so can't really compare to the others.
3. I don't think prepurchase is necessary. If you prepurchase, you save a couple euro off the price (there are other discount options too). If you have more than a few people, it probably adds up enough to take advantage of some discount option.
4. Vadettes du Pont Neuf is on the downstream end of the Ile de la Cite - the same island as Notre Dame but the other end, toward the Eiffel Tower. When you get to the end of the island there are stairs down to the small park, go down the stairs, turn right and you'll run into them.
5. Any time after dark you will see the Eiffel Tower, whether it sparkles right when you're next to the Tower is more than I can figure out. Maybe if you depart about mid-hour, you will be up at that end at the top of the hour when the light display happens?

We were at the Eiffel Tower in the late afternoon/early evening and walked across the street and down the stairs to the river. There are a couple of River Cruise companies right there.
We bought our tickets out of a machine and got in a tour that started in about 30 - min..
Boarded the boat and had a choice of sitting inside or outside on top.
The 1 hour tour sailed down to the Ile St. Louis - around the backside of Notre Dame,
turned around and sailed back to the start.
It was beautiful night and we got to enjoy the lights and sights along both sides of the river.
Very easy, no reservation, no hassle and short wait.
It was February and cold , but we sat outside all bundled up and loved it.

The Eiffel Tower only "sparkles" for 5 minutes on the hour, after it gets dark. There's no guarantee that any boat will arrive during this time frame.
If your daughter wants to get a close up view when the ET sparkles, you should choose www.bateauxparisiens.fr which docks very near the base of the Eiffel Tower, and be there on the hour, then take the cruise. You won't see too many monuments from the river's level, but the bridges and monuments are illuminated.

If you miss the 5 minutes of sparkle from the boat, there are two other great places to see it. One is from the Trocadero, which is somewhat elevated, so it gives a great view of the Tower. This is free. The other is the top of the Arc de Triomphe; this is included in the Paris Museum Pass, and you have to climb it, but the view is worth it, day or night.

Thanks to everyone. I appreciate your suggestions.