Contrary to what people often think, children can in fact be dragged into museums and cultural venues while on holiday. You just need to work in some playtime in between to compensate. If you are spending more than just a few hours in Dresden and have a car, I may have a nice little solution for you that in fact all ages will enjoy. In less than an hour’s drive is a perfect little rock-climbing area that is natural, un-commercial and unspoiled (http://www.wandern-saechsische-schweiz.de/Steine_Labyrinth.asp German only, but includes GPS data).
Just take the road through Pirna (Route 172). Once you’re through the city, you come to a turnoff toward Rosenthal. As you go down this road, you will see signs for the “Labyrinth”. Follow those signs to the parking lot, get out and walk for 10 minutes and you find yourself in another world. Children will love to scramble all over the rocks and you can picnic there too.
If you plan your trip well, you can even include a visit to Königstein, Europe’s largest fortress and interesting as well as beautiful, as it is only about 10 minutes farther along. It is in fact the large structure that you see on the top of the rock on your way to the Labyrinth. Please remember, though, that you are in fact in a national park and need to act accordingly.