The Tsar of Bulgaria from 1943 to 1946 was a child (6 years old at accession) called Simeon. After the war, Bulgaria became a communist republic and Simeon fled, but returned to the country in 1996, went into politics and was elected president in 2001, serving until 2005.

Edit; Post-1994 I realise, but I thought it was interesting. How relevant his status as "reigning" Tsar was to his election I don't know.