With kids, it depends on the day.
The Sedlec Ossuary — a chapel chandelier and coats-of-arms made of human bones — is genuinely unforgettable, and St Barbara's Cathedral plus the old town make a satisfying full day. The catch: the bone church is small and now timed-ticket and photo-restricted, and it's a short walk from the train but a longer one to the cathedral, so plan transfers. A strong macabre-history day out.