I went to Gdansk and Lithuania last year. Gdansk I think would be good for slightly older kids with the shipbuilding and all the WWII history. Outer banks of Lithuania is pretty and reminiscent of Wisconsin but with just a bit more ocean. Lots of nature-stuff there. Not entirely sure what kids would do in Vilnius, though I also never looked that up.
edit: Gdansk has a terrific museum on the Polish Resistance and memorial to the shipbuilders killed by the Soviets. I didn't have a chance to see the spot where the Germans first started WWII, but there's a sailed ship museum there, as well, which looked really interesting. Also an outdoor opera house tucked into the woods. Lituania's outer banks was a Viking settlement and a tourist spot for biking and enjoying the nature reserve there. Vilnius is such a beautiful city. Museum of Occupation was very good and there's an Illusions museum I'd guess the kids would like but I didn't see. Oh, also near Gdansk is the largest castle in Europe, though I most enjoyed sitting near it and just looking at it with some food and beer in my hands.