Around 12-13 hours flight from HK to Germany.
Weather is very fine in Summer, not too hot.
Summer is day long night short, still have sunlight at 7-8pm
If you only have 1 week and have not been there before, I would suggest you stay in the Southern part of Germany. Munich is a convenient point for trains and you may consider staying there for 4 to 5 days. In there you can vist Deutsches Museum and BMW Museum (boys like them a lot). Also, you can travel to Fussen (probably day return) and visit the castle - 新天鵝堡.
I have not compared the size of Legoland in LA and Germany, but the latter is quite large and the first Ninja land is built in Germany so you can imagine how the quene is for the flying Ninja game. Ulm is only half an hour train from Legoland (Gunsburg) so we probably will stay there for 2 days.
In the previous two summers, we travelled to Germany covering Wunsburg, Nurmberg, Fussen, GP, Munich, Frankfurt, Berlin, Dresden, etc as well as Saltzburg (Austria), Praha and CK (Czech). Germany is a safe and good place to travel with kids.
I love Germany!!! However, I haven't been to Germany with my daughter. I wonder she would be able to enjoy travelling in Europe yet or not.
How about Europark?