Would love some advice on when to go in 2015. Taking my 6 year old, 4 year old, and 1 year old along with wife and mother in law. We go every other year, normally in September. We love the low crowds, but it can be a little warm. In November, we have an opportunity to go the 14-21 with a small home school group. We have never done the Christmas party, and would love to. But what about the crowds? Will it be that major of a difference between September and November? Any advice would be helpful! Thanks!
The Historical Crowd Calendar can really give you a good idea of what to expect - the crowds will definitely be higher, but still on the low side.
The crowds are slightly higher in November, but the weather is more predictable (not as hot and not as much rain…usually). We try for late October or early November usually.
November has Christmas party and decorations. We just got back, and they were great. if you have never been, I highly recommend trying it during festive season at least once.
If you are used to sept crowds… You will feel like it’s super crowded in nov! We did that one year and I know it not like holiday crowds but more than we were used to before… Still had a great trip though 
end of August/early sept are my fav times for crowds… Yes it feels like you are walking around in the sun but at least no one is around! We do have a new baby so I won’t be going in the heat for a few years so I would say for you, brave the crowds in November for the weather!
Personally, I really don’t like the misery of September heat. I think November is great, even though there are higher crowd levels. I’d rather stand in slightly longer lines in cool weather. My kids are 6, 4, 3, and 1, and I just took them Friday for a day trip by myself. The weather made everything so much easier! I wasn’t stressed about keeping them hydrated, they only complained in one line all day, and we made it 8:30 - 6:30 with no breaks. That would not be possible in September.
It is personal though. I know there are many people who will endure the heat for the shorter lines. It just depends on your tolerance, I guess.
I would vote for November, especially if you would like to do MVMCP. Better weather is a good trade for lower crowds in my opinion, and if you have a good touring plan, that will help counteract the crowd effects.