The Sheraton in Krakow seems to get decent reviews and starts at Euro145 in the summer, which is not too bad. The Le M in BUD is very nice, but quite old world and can be hog whimperingly expensive.

There is a Le M out of town in Vilnius, but I've never been there.

So, perhaps Krakow is the option? Lovely place, I imagine the kids will enjoy eating in a cellar ( ) and it has enough big city buzz without being overwhelming.