I wasn't sure it would be worth the money with a small child, but it totally was worth every penny for us! We went on October 27th last year which was a Sunday night. We spent the morning relaxing by the pool at our resort, then took a nice long nap. We got to MK around 5 pm or so and had dinner before the party officially started at 7 pm.
We went into the night with a plan, prioritizing our must-dos, including meeting Mickey, Flynn & Rapunzel at Princess Fairytale Hall, and seeing the Boo to You parade & Hallowishes. We ended up doing all of those things and more! Surprisingly, it wasn't a sold out night despite being so close to Halloween, so I'm sure that helped. We had not planned on doing any Trick or Treating but we all three came home with bags full of candy.
The parade and fireworks are absolutely stunning, and worth the price of admission to the party on their own!
Our son was newly three years old at the party, and I had stressed myself out planning out how we were going to do so much before he presumably fell asleep at 10 pm or so. I was kind of happy to be so wrong about that! He ended up staying awake for the whole party and enjoyed every minute of it! We did call it a night a little early and leave at 11:15, but I think he would have lasted until midnight if we had decided to push it a bit later.
We had a lot of fun dressing up, too! DS was Peter Pan, I went as Tinkerbell, and my DH went as Michael. So needless to say, he got a lot of fun reactions to his costume!

I would totally recommend dressing up, it makes the night so much more fun and festive! But be aware that at the end of September it will still be pretty hot, even at night after the sun goes down, so I would plan costumes accordingly.