India Roy and Isla Lambert, both six, had eight inches chopped off and will donate their locks to charity the Little Princess Trust.
Trixie Pook, nine, attends Hayley Sexton School of Dance in Portsmouth with India and Isla.
Earlier this year, she was diagnosed with germinoma, a tumour found on the brain. She immediately started radiotherapy and, three weeks into the treatment, started losing her hair.
When India and Isla heard about Trixie’s ordeal and the wig she had been given, they decided to help.
Natalie, India’s mum, said: ‘When India and Isla were told Trixie was going to lose her hair, they offered to give her theirs. We looked into it and found out about the Little Princess Trust who would make their hair into wigs.
‘Both of them were really keen to do it. As well as donating their hair, generous family and friends have raised more than £1,300 to cover the costs of making the wigs.’
India and Isla, who both attend Westover Primary School in Baffins, had their hair cut at David Anthony Hairdressing, in Milton, on Saturday. Both were excited to see their new bobs.
Isla said: ‘I feel happy because I am doing something good to help another little girl.
‘I have never had my hair short so I think it will look a little weird. But I am excited.’
India added: ‘I am happy to cut my hair off for Trixie.’
Trixie’s mum Heidi said it had been a whirlwind few months since the diagnosis.
She said they noticed Trixie was drinking a lot and tests in February revealed a problem with her pituitary gland, which controls hormones.
An MRI scan revealed the problem with the gland was due to the tumour which was putting pressure on it.
Heidi said: ‘They needed to get the tumour out so she had brain surgery. Because there were smaller bits, she needed to have radiotherapy too.
‘Luckily, this is a highly treatable kind of tumour and we are waiting for an MRI to make sure it’s gone.’
She added: ‘When I was told what India and Isla wanted to do, I started crying.
‘Trixie’s confidence has been knocked by this so for India and Isla to want to help is amazing.’