More than one million children have fled Ukraine since Russia invaded on 24 February, according to the UN's children's agency.
UNICEF spokesman James Elder described it as “a dark historical first" on Tuesday.
It means that children represent around half of the more than two million people that have fled the war, an exodus that the UN's refugee agency UNHCR has called the fastest-growing refugee crisis in Europe since World War II.
Ten-year-old girl Annamaria Maslovska is one of them.
After bombs started falling in her hometown of Kharkiv, she left her friends, her toys, and her life in Ukraine and set off on a days-long journey with her mother toward safety.
After finally crossing the Hungarian border by train along with hundreds of other Ukrainian refugees on Wednesday, she told AP news agency that she'd begun to worry about her friends in Kharkiv after the messages she sent to them on Viber went unanswered.
“I really miss them because I can’t contact them, they just read my messages and that’s all. I really worry because I don’t know where they are,” Annamaria said in clear English from inside the train station at the border town of Zahony.
She and her mother, Viktoria, planned to travel on to Hungary's capital, Budapest, but didn't know where they would go after that. Annamaria said she hoped to visit Disneyland in Paris.
Since men between 18 and 60 are required to stay and fight in Ukraine, many families have been fleeing without their fathers and husbands.
And neither the children nor their mothers know when - or even if - they'll be reunited.
Once the war ends, Annamaria said, she wants to go back to Kharkiv and reconnect with her friends who have been scattered by Russia’s violent invasion.
“If war stops, I really want to go home because there are my friends, there are beautiful parks, supermarkets, centres, and playgrounds behind my house,” she said. “Kharkiv, it’s like a piece of your heart.”
Some children are also fleeing Ukraine unaccompanied by parents or adult relatives and they're particularly vulnerable, according to UNICEF.
Most of the adults and children fleeing the war have entered countries on Ukraine's western border, including EU states Hungary, Poland, Slovakia, Romania.
The majority have reached Poland, where 1.33 million refugees have crossed according to the Polish Border Guard agency.
"Poland receives 150,000 a day alone. It's going very fast, we don't see this movement stopping," UNHCR's head Filippo Grandi told a news conference in Stockholm on Wednesday.
UNHCR has warned that, whilst the first waves of refugees had family and friends to link up with and support them, mostly in Poland and Romania, it's likely that later waves will be less supported and more desperate.