Sundays in the Retiro Park are great for kids. Lots of kid-friendly and kid-focused entertainment there - but not sure for 11-year olds. Maybe renting a rowboat in the lake would be fun for him, though. Kids tend to love the Teleférico from Parque de Rosales to the Casa de Campo, too. And Crisco's right, don't forget a Santiago Bernabeu Stadium tour. That's pretty cool.

Saludos, MadridMan