She has become the youngest person in the world to be fitted with the flex-run feet used by leading paralympians including Oscar 'Blade Runner' Pistorius.
Ellie, who lost her hands and lower legs to meningitis as a toddler, had spent the last two years walking on conventional prosthetic limbs. But she complained that she struggled to keep up with her twin sister Sophie.
Her parents Paul and Lisa contacted the world-renowned prosthetic limb centre Dorset Orthopaedic, and managing director Bob Watts agreed to make a junior version of the carbon-fibre blades.
Mr Watts said: 'We were worried that she wouldn't be able to balance properly on them, but she has made amazing progress.'
Ellie, from Little Clacton, Essex, was struck down with meningitis at the age of 16 months.
At one point her heart stopped and doctors called in her parents to say their last farewells.
'Paul and I just stood at the end of her bed shouting at her to survive,' said 35-year-old Mrs Challis, who works as a carer for adults with learning difficulties.
'We thought we were going to lose her, but amazingly she pulled through and her heart started again.'
Over the next four days, however, Ellie's legs and arms started to turn black and the couple were told they would have to be amputated.
'The operation took six hours and I was just so shocked when I saw her - there was so little left of her,' said Mrs Challis. 'I just kept crying.'
Ellie was inspired to overcome her handicaps by Sophie, her constant companion. And well-wishers raised hundreds of thousands of pounds to help her.
She was fitted with NHS prosthetic limbs a few weeks after the operation, but found them too painful.
Mrs Challis said: 'She would wear them for 20 minutes maximum each day as they weren't flexible and they caused her pain.'
In December 2006 Ellie was given new legs with moving knee joints, which were a great improvement.
But the flexible blades will change her life. They helped South African Pistorius race to world records in his 100, 200 and 400-metres events.
Mr Challis, 45, said: 'Ellie can walk twice as fast on these new legs - it really is amazing to see. She is so full of determination.'
The legs cost £10,000 for a pair and will need to be replaced every two years. Mr Watts added: 'We didn't know if it would be possible to make some small enough.
'But now Ellie is the youngest in the world to have such legs - and there is no holding her back.'
