My first and most important comment would be that the greatest gift you can give your kids is take them on as many international trips as possible. It will absolutely change them... in a great way. Since it's your first trip you might want to consider an English speaking country, just for the comfort and ease. You might find having to deal with another language overwhelming. If not then any of your choices are good ones. We've done all your listed locations, except for Frankfort. They're all great, and have unique things that are great experiences. Happy to provide and specifics if you have a particular question.
Correct. It will still be cheaper, but getting more expensive every day. At least our deposit for our apartment will be worth more when we get it back in a few years... unless we decide not to come back.