We went to
MNSSHP this past September, and it was the most crowded we have ever seen it. In fact, it was so crowded that we felt like it's probably the last time we will go to that event.
I understand why she wants to go - it's a lot of fun, but as others here have said, the meet-n-greet lines are typically long.
Aside from the meet-n-greets, is there anything in particular that is compelling to her about the party? The decorations of the Park are the same whether it is a party night or not, and with the party, there are some attractions that will be closed (it's slightly differs every year, but for example, Tiki Room is almost always closed for MNSSHP). Many of the QS spots (and the Table Service restaurants) close up early as well, so be sure to eat dinner while you can.
Aside from the Castle stage show and the parade (best ear worm ever!) which you can watch any time on YouTube, the biggest draw for kids tends to be the trick-or-treating. Disney does give out good candy

but hitting every spot on the candy trail can be tiresome, and often you find that you are shuffling along in a line, waiting to get to the CMs with the candy.
Would she perhaps have more fun (for about the same money) if you all did a Fireworks Dessert Party instead? Just a thought, based on our most recent experience.