Milleigh (7) and Max (4) Milner are running and cycling 100 kilometres to raise money for ocean-therapy charity Healing Waves.
The sibling duo, dubbed 'Team M&M', are five days into their 10-day challenge and have already hit the 50km halfway mark.
They are hoping to get enough donations to buy a new sensory swing for the water sports charity.
Mum Holly Milner says the idea came from talking to Max about what charities are.
"We we're playing on the beach at Le Braye and Max saw the new centre that Healing Waves have, and he had a lot of questions.
We explained how charities work and that they relied on people making donations, so we came up with the idea to do a challenge to help them."
The children have been speaking to Channel 103 about what they wanted to do to raise money:
"I wanted to do cycling - on the hills I am very fast" -Max
"And I wanted to do running because I'm not that into cycling and I'm quite good at running" - Milleigh
The pair have already surpassed their £3,500 target and increased it to £4,000.
Holly says Healing Waves has helped their friend's children and that's why it is so close to their hearts.
"We've seen photos of my husband's god-daughter using the adapted surfboards, so it's been lovely showing the children a charity that they can really understand and see that they're helping."
Max Wiltshire, who is the charities says the challenge highlights the siblings character.
"We're absolutely blown away with them, they're a force to be reckoned with. For us it's really touching that the younger generation looking to Healing Waves and wanting to support us"
The charity hope to utilise the space at their new Le Braye centre.
"We're looking to make it a really good sensory haven with nice plants and all things to attract butterflies and bees so we've got things to give off all sorts of scents and sounds as well as being really colourful."
The brother and sister dream team finish their challenge on Monday 5 September.
You can donate to their crowdfunding here.
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