Jo Cullimore, 46, from Waterlooville, Hampshire, has been helping children from the town of Borodyanka in northern Ukraine since 2012.
Russian forces invaded Borodyanka, just outside the Ukrainian capital of Kyiv, on February 24 but have since left the town.
“It’s heartbreaking and every day I wake up thinking I’ve just dreamt the war, but then I watch the news and remember that it is real,” Ms Cullimore told the PA news agency.
“I started volunteering for a charity in the UK and then went on to helping take aid out to the community and Chernobyl children in 2015, with the help of my mother Lindsay and my 13-year-old daughter Emma, who is now 18.
“We spent a lot of time working in the town of Borodyanka and the children would put on performances at their school for us and we would help with English lessons.
“The Russians have recently been in that school and now it’s no longer useable, they’ve even graffitied all over the walls, it’s just horrible.”
Before the war, Ms Cullimore, who is a mother-of-two, had already formed close bonds with the families she had worked with.
She has remained in constant contact with one mother in particular, Iryna Berdnikov, who is also a mother-of-two and has a teenage son who is the same age as Ms Cullimore’s 17-year-old son.
Before the coronavirus pandemic, Ms Berdnikov’s son Illia visited the UK to spend a month with Ms Cullimore’s family.
Ms Berdnikov, her son and her four-year-old daughter are now staying in Poland after fleeing their home in Borodyanka, but the children’s father Sergei remains in Ukraine.
Ms Cullimore makes frequent visits to Poland to help around 40 families, some of whom are in Poland and others are still stranded in Ukraine.
“I have a particularly close relationship with Iryna, to the point where we now call each other sisters,” Ms Cullimore said.
“When Illia’s younger sister, who is now four, was born I went over to help them and I have recently spent a lot of time in Poland with the family.
“But the trauma… when I speak to Illia now, he says he’s fine but I know something is clearly distressing him.
“Especially when his mum goes to call her husband, who is still in Ukraine, and comes back with tears in her eyes… it must be so hard.”
Photographs sent to Ms Cullimore, and shared with PA, show the destruction of the school in Borodyanka where she used to volunteer.
The images, taken by the headteacher’s husband – who is a friend of Ms Cullimore – after Russian soldiers left, show the bright yellow walls of a children’s classroom covered in spray-painted Russian words, and wooden desks littered with empty bottles and shattered glass.
“The people I know who are still there have told me the town has been as destroyed and flattened by the Russians,” Ms Cullimore said.
“The town is basically one long street, with a few houses behind.
“They drove their tanks down shooting at everything and there is nothing that hasn’t been damaged or destroyed.
“They didn’t destroy the school totally as the Russian soldiers used it to sleep and live in.”
Poland has received more than three million refugees from Ukraine since Russia launched its war on February 24.