1. I can't answer your first question for certain. We went Oct. 22nd last year, and it was a Friday and quite busy. But it was also during F&W, and closer to Halloween. My inclination would be toward Sunday in terms of crowd level, but the dates are very different from mine last year.
2. Not sure about #2, except as I understand it, most things continue on if it rains. Be sure to take ponchos (you can get cheap ones at
Walmart). But I don't know if it's the same for
MNSSHP.
3. Our dd was 3 last year, and there really wasn't anything scary about it. It was a little darker in the park than other evenings, but not by much. If they have any issues with darkness at night, that's the only thing I could really see as scary.
4. I definitely enjoyed it and would do it again if I went during that time of the year. It was one of our favorite activities of the trip. However, it might be tricky with a 2 and 4 year old. To get your money's worth, the later you can stay the better. In our case because crowds were fairly high, we didn't necessarily do more during that time frame. My dd was most interested in the actual trick or treating, which was fine. We enjoyed just wandering about and checking out the decorations, but I can imagine that for some, it would seem like a waste of money to do that. So, I guess it would really depend on your kids and what kinds of things you think they would enjoy about the experience.
Good luck with your decision!