In Madrid for about a day and a half, what are the must-sees

Palace Real and pop into the Cathedral across the plaza from the Palace. The Prada for the Old Masters and/or the Reina Sophia if you like newer art. A walk in Retiro Park. If you time your visit to the Palace Real you should absolutely go to the Temple of Debod for sunset. It's the only time I have ever been in a crowd of people that actually burst into applause once the sun had set. Walk down to the overlook for the sunset if you go. Walk around Peurto de Sol, lots of fun stuff for the kids there - mimes, living statues, jugglers - but be aware they expect to be paid if you take pictures with or of them. While there you simply must stop in at San Gines for hot chocolate and churros: the hot chocolate is so thick you can stand your churros up in it. You can then walk to the Plaza Mayor. Stop in at La Torre del Oro. It's a teeny tiny bar but your kids can go in. The walls are covered with gory bull fighting photographs. I found them fascinating, dh did not I bought bottled water there because I felt bad spending so much time inside without ordering anything.

And not that you asked, but if you have time in Barcelona, you should definitely visit the Sagrada Familia. Buy your timed entrance tickets from home before you leave and pay extra for the guided tour and, if you're not afraid of heights, the walk across the bridge between the two turrets. If you don't buy tickets in advance you can wait hours to get in instead of walking right up.