I'm of two minds about taking little ones to the MHP. Tickets are expensive and it's hard to justify the cost if you aren't going to get to stay the entire time (either until midnight or 11:00 pm depending on the party night). There is so much to do during the party (e.g., the parade, fireworks, trick-or-treating, characters, along with attractions). I went with an almost three year old several years ago and it was fun...but he did start to struggle to stay awake about the time that the fireworks were slated to start. He made it through those, but he crashed soon thereafter. So one adult ended up having to stay with him, while the others (and his older sister) went off to do more fun stuff. We were lucky because we had enough adults that we could rotate through who stayed behind so no one really missed out on much, but if you only had one other adult, I could see where it would be tough to justify the cost.
In terms of timing, as others have mentioned, HalloweenTime is very popular in
Disneyland. They have not yet announced the starting and ending dates for 2017, but I think it's a pretty safe guess that it'll go through at least Halloween night. They haven't announced the start of the Holiday season, but that could potentially start as early as Veteran's Day weekend (based on recent history). I do think the weekdays (Tue-Thu) stand a chance of being okay, but I kind of doubt there will be a "low crowd" weekend from mid-September (or whenever HalloweenTime starts) through mid-January (when the Holiday season ends).
Nov 4/5: Dapper Days
Nov 9-12:
Super Heroes Half Marathon, Veteran's Day weekend (potentially the kick-off to the Holiday Season)
Nov 23/24: Thanksgiving