Если ребёнок в возрасте до 18 лет путешествует с одним из родителей, нужно получить нотариально заверенное разрешение от другого родителя.

I just witnessed the unpleasant sceen in Sheremetievo airport where a Grandmother was crying over customs counter and could not take her 12 year old grandchild abroad with her (cause she had no notary certified permission from BOTH pearents!!!) Please be carefull - the consent of the other parent is NECESSARY when you go through passport control in the airport. Call to Sheremetievo to enquire for their confirmation of what I said, and I meanwhile will find the Instuction that requires a consent from the second parent.