We were at MK for last NYE. Me, DW, and DD4. IMO the crowd wasn't too bad throughout the day. (DW is a teacher and we end up going at all the peak times, in our experience there was a day at AK during the week before Easter that you could not move at all, this wasn't that bad) Anyway, the crowd increased as the day went on and following the first showing of the MSEP at 7:00pm, it was finally packed as we made the "mistake" of going back to Main Street.
People had staked out their spots for the night and weren't moving. CMs were everywhere directing us to move, but not to anywhere in particular. If you didn't have a spot staked out, you had to keep moving, and they basically direct you around in circles. It was frustrating. We finally stopped listening to the CMs, and we literally went back against traffic and broke out of that traffic jam and escaped into Adventureland.
We walked all the way around to Fantasyland, where it was MUCH less crowded. We spent time on rides in Fantasyland and Tomorrowland and we were back in Fantasyland, right between the carousel and dumbo for the fireworks at midnight.
The fantasy in the sky fireworks started at 11:50, and ended just before midnight. Then mickey and the gang come on and count down to midnight. The 10 second countdown had fireworks to go with it, and then the sky exploded all around us. It was cool!
We could have never staked out a spot and waited 4 hours in front of the castle or on Main Street like so many people were doing, not with a 4 year old.
Since it is just the two of you, that will make it easier for you to navigate the crowds. My advice would be not to bother staking out a spot for the fireworks, you can see them from anywhere in the park. Just make sure you pick an open spot, so you are not under a tree or right next to a building, so you can see the sky all around you.
Following the fireworks, we headed back toward Main Street by going around the castle through tomorrowland. At Cosmic Rays they were giving out free hot chocolate and cookies! We thought that was a nice touch. Back on Main Street, we got to enjoy the christmas decorations and lights on our way out. It was even snowing!!