In accordance with ą .6 "Agreements between the Republic of Belarus and the Russian Federation on ensuring equal rights of citizens of the Republic of Belarus and the Russian Federation for freedom of movement",the choice of the place of residence and residence in the territories of the states-participants of the Union State "citizens of one country can leave for the third countries (in this case, Ukraine) from the territory of another country in the presence of valid documents, issued in accordance with the law of the state of citizenship for traveling abroad. The ,unless otherwise provided by international agreements of the Parties

Simply put, an adult passport is sufficient for a citizen of the Russian Federation, a child has a birth certificate with a mandatory insert or a stamp confirming Russian citizenship.