Most of the cities "blessed" with GDR architecture in a dominant way are not considered to be the prettiest. Anyway, since you are in Berlin, Potsdam is an obvious choice. Other cities in the region which could interest you are Cottbus, Eisenhüttenstadt, Frankfurt an der Oder or Neubrandenburg. Though I'm not sure if these cities are fitting for a longer stay or holiday with two little kids.
If it is not too far away, I would recommend Rostock on the baltic see. Hotel Neptun
was quite famous (and unreachable) during communist times. The city is nice and provides quite a large range of GDR architecture from hanseatic shaped stalinism
to extended Plattenbau quarters of (at least IMO) higher quality than in many other cities and postmodernist examples
from the 80s.
PS: Dresden is of course always worth a visit. You could stay at Hotel Bellevue, built in the 80s incorporating a baroque building and garden.