It's one of the few cities in Poland not systematically reduced to rubble. It's a gem, with one of the largest medieval squares in all of Europe. Great churches, wonderful castle, great cafes. Gorgeous.
An hour a way is Auchwitz-Birkenau, which I think every person should see in their lifetime. I'm not sure what the appropriate age is for kids, I'd want mine to be well into middle school, I think.
Warsaw is kind of the best and worst of Poland in some ways. I think the Old Town is also charming, and the palace there is a great tribute to the absolute stubbornness of the Polish people. It was dynamited to a pile of rocks by the Nazis. Afterward, people came out and collected little bits of marble, molding, etc., knowing at some time later they would rebuild it. Going into a room and seeing a few old weathered pieces of trim which were used to re-create the entire place is pretty incredible, frankly.
Much of the rest of Warsaw was build under Soviet rule, and follows the predictable and bland architecture found behind the Iron Curtain. In a way, this can be interesting in and of itself, but outside of the Old Town, it ain't Prague.