I think this question really varies depending on if you've been there or not. It's also true for adults, especially ones that think wdw is a waste of time and money. The honest answer is you need to know who your going with and plan according to them.
For myself and my daughter, it never really felt like we were at disney until you stand in front of the castle. I call it the "we made it loop" around mk. We also go right in on travel day. Sure we get a fastpass or two but the loop isn't to accomplish anything. We'll circle, maybe do some rides, definately get a snack or meal in a specific spot that is special for us and then we are officially on vacation and started. With that said, it really does impact the other parks because they just have less to do than MK and it's not as easy to do them with our style. We have tried other parks because there is some favorite places for table service so we do that right away but it doesn't have the same impact as getting to mk and declaring you made it back.... We generally end up back at mk every day or almost every day park hopping so I guess for us, it does impact us when you really think about it.
It's also become very hazy, especially for younger kids. You have the frozen is everything fans and there really isn't much frozen at MK. Then you have the action movie/star wars lovers and HS is their park... I find AK the most annoying to walk around with the way the paths go up and down more and after being in some parks where you can hide in A/C to cool off easier, it's rough to do that park so one might say if your in florida in the hot and humid season, AK might be the best to start while your not sore and not adjusted to how to be in the heat yet...
Epcot I find to be one of the least liked parks by kids (and my daughter still hates it as an adult). They are at disney, so starting there might not be a great move but getting there sooner than other parks can be a good thing. After mk can be rough for epcot so you need to space it with another park they will like that takes them down from all the things mk has to do first..
Honestly, the only way to know if mk would be better not being first is to try it.. just be ready to possibly declare it a mistake and hop to it or make sure it's day #2 when you first try it[/QUO
Exactly. When we went with our five, they had never been there before. (In 1995). Once you do MK, nothing else compares.