No "special" documents are required. Importantly, the documents must belong to the bearer and be valid.

If children are accompanied by both parents, for unimpeded entry into Belarus, only originals of birth certificates will be needed, with the mark that they are citizens of the Russian Federation.

Instead of a birth certificate, you can use a special travel document that is equivalent to an adult foreign passport. If only one parent accompanies the children, in the case when all members of the family are Russian citizens, a permission from the second parent for the child's departure officially certified by a notary is not required. Russians can travel around Belarus with almost no restrictions, both on the territory of the Russian Federation.