I personally think that MK would be the best for NYE if you have kids. You can still see the parades, they do fireworks at 10:00pm (I think) and the big ones at 12:00am, you can deal with waiting for 1/2 hour to do something like Small World. I definately think MK on NYE is the way to go. If the family gets tired and cranky- get on the train for a couple of loops. Toon-town is rarely packed. You might even be able to do Barnstormer. Plus- they have the new playground by the Pooh ride so the little ones can just hang/play.
MGM just doesn't have enough to thrill me to make it an option. The only good rides would be mobbed and your little ones prob. can't do them anyway.
The EPCOT fireworks are cool. They do the regular show (Illuminations) and then they do this cool beginning to the wild midnight part. They acknowledge that the countries experience "Midnight" in different time zones. They light up each country in the order they celebrate it and do a mini firework behind it. Then they do the final countdown. It's cool and if you're staying at BC, BW, YC, Swan, or Dolphin- it's the easiest to leave and walk back to within 5 minutes. However- if you're talking about a long day in the park and entertaining 4 kids- go MK. Trust me- if you head to EPCOT on NYE- the kids will be bored/complaining.