Since the abolition of entry visas for Russian citizens, no special documents are required for the child to enter the country. List of required documents:

• a passport with a validity of at least 6 months from the date of entry, or the child must be inscribed in the passport to the parent with whom he leaves.
• Birth certificate
• medical insurance
• an invitation from the people they are traveling to.

In addition, now it is officially possible not to have a power of attorney for the child's departure from the second parent. But, my personal advice still have a power of attorney to myself.