The Russian leader, 69, is said to have been in a secret relationship with Olympic gold medallist Alina Kabaeva, 38, since 2008, and the couple share two children.
The former gymnast and former Duma lawmaker is closely associated with President Vladimir Putin, the EU said in a document laying out a new round of sanctions on Russia.
The first boy was born in secrecy in Switzerland amid huge security in 2015 and the second son was delivered in Moscow in 2019 with the same specialist flying to Russia for the birth, reports say.
Alina Kabaeva, 38, stepped out to lead her eponymous Alina Festival at the Russian capital s VTB Arena, putting paid to rumours she was hiding in Switzerland or a bunker in Siberia.