Not sure why you want to go to Legoland in Germany. Either go straight to Denmark or go to Windsor, the German one is no different to the UK one.
We (we are Germans) always fly unless we have to take the car to transport large items. You can fly direct to lots of areas, and while you can take a train it takes around 8 hours, I did it due to plane issues and won't recommend it for an adult, less for somebody travelling with children.
Themeparks in my opinion are always the same, I can't really see a reason to just go to Germany for them. Germany is a great country for holidays but choose an area you want to see not to tour themeparks.
We have a bit of everything, sea side (North Sea and Baltic) plus lots of lakes, mountain (medium to alpine), historical towns, castles, outdoor adventures.
I assume you mean the toy museum in Nurnberg, you would spend all your time in trains or in the car and not actually seeing the country. Phantasialand to Nurenberg is around 4 hour train ride from there to Legoland anonther 4-5? with various changes.