In the area indicated which encompasses half of Germany, Austria and Switzerland and all of Lichtenstein, there are:

Many mountain peaks accessible by cable cars (or whatever the American word is): Great for kids of that age!

You can take boat trips on lakes (e.g. Bodensee/Lake Konstanz): Great for kids of that age!

You can take small train trips up the mountains: Great for kids of that age!

You can go see the Rhine falls: Great for kids of that age!

Castles up the wazoo, from original medieval ones to fake medieval ones like Neuschwanstein: Great for kids of that age!

Zoos, when in big cities: Great for kids of that age!

Yes, many places may be crowded in midsummer, but there is nothing you can do about that, other than re-scheduling your trip, which you will certainly not be able to. You'll also notice that the things that I annotated with ": Great for kids of that age!" are also great for grown-ups