Parents hold children brought back to Ukraine after being brought to Russia from territories occupied by Moscow.
Mykola Kuleba, head of the charity Save Ukraine, said: “We are welcoming 31 more children who were illegally arrested.”
Mr Kuleba praised the “heroic mothers” who saved their children.
Kiev says more than 16,000 Ukrainian children have been deported to Russia since the invasion.
Russia denies the allegations.
The children were brought in from the northeastern Kharkiv region of the pro-Western country and the southern part of the country. KhersonSave Ukraine, which fights what it calls illegal deportation of Ukrainian children, said.
On Friday, the charity said the children and their relatives had crossed the border into territory controlled by Kiev.
According to footage released on Friday, the children with suitcases and bags walked across the border and then boarded a bus to continue their journey.
An elderly woman who was supposed to bring her two grandchildren home, Kuleba said, died from “stress”.
write on Facebookhe said pro-Ukrainian relatives had been “interrogated for 13 hours” by Russia’s FSB Security Service.
Last month, the International Criminal Court issues arrest warrant for the President of Russia Putin above War Crimes allegation of illegal deportation Ukrainian children.
The Hague-based court has also issued an arrest warrant for Maria Lvova-BelovaRussian presidential commissioner on children’s rightswith similar fees.