During her MDNA Tour, one of Madonna's children joined her on stage. Who?

Explanation

The american singer-songwriter Madonna has embarked on nine concert tours (anno 2013), eight of which have been worldwide. Her debut concert tour was called "The Virgin Tour" (1985). After that she follow up with "Who's That Girl World Tour" (1987), "Blond Ambition World Tour" (1990), "The Girlie Show World Tour" (1993), "Drowned World Tour" (2001), "Re-Invention World Tour" (2004), "Confessions Tour" (2006), "Sticky & Sweet Tour" (2008-2009), and "The MDNA Tour" (2012).