I think you might have fun in Strasbourg-that is about 4 hours drive. It is a nice walking city with canals and nice places close by like Colmar.

Nuremberg to Vienna in just 4h15 by direct high-quality ICE, book at German Railways www.bahn.de/en from just €29

Nuremberg to Prague in 4h43 with 1 easy change at Cheb, from €15 booked at Czech Railways www.cd.cz (Important: Click 'More options' then 'Travel via' and put 'Cheb' in the 'via' box to get this option and this price)

I don't know how kid-friendly Vienna, Prague and Strabourg would be for 3 children under 6 years of age, but if you wanted some outdoor activities, like lakes, forests, and mountains in a renowned summer holiday destination, you should consider the Salzkammergut region of Austria, around Salzburg. It is about 3 hours from Nurnberg.

https://www.salzkammergut.at/en

We quite like Vienna for kid-friendly activities as we took our daughter there twice when she was under 6. Some of the things you might see:

*the butterfly house at the Hofburg; have an ice cream on the lawn outside afterwards
*the Zoom Kindermuseum
*the aquarium
*the Prater - giant ferris wheel, pony rides, etc.
*cycling through the city - you might be able to rent a 'trailer' that you put kids in (we didn't do this as we only have one child, just rented a bike with a child seat)
*our daughter liked churches and ensuing theological discussion (we are not religious but it fascinated our little one)
*the summer 'beach' on the Danube

There is also a zoo at Schönbrunn that might be interesting but we didn't get there.

What didn't work for us so well in Vienna is the cafés - there is a lot of child-friendliness in Vienna but not in its famous cafés, where they don't expect to see children. However there are enough places where you can take your child (casual restaurants, Konditoreien (cake and pastry shops)). We also would have liked to get to a Heuriger but it just didn't happen, and I think that would have been OK with kids too.

Good luck!

Lavandula

I would second the suggestion of the Salzkammergut region of Austria which seemed to us to be kid friendly.
Your children are perhaps a little too young at present but in the future you might want to consider rafting down the Vltava River around Český Krumlov.