The daring duo from Wem have already met up with presenter Matt Baker for a walk near their home and will join him in the Lake District this weekend.
They are supporting Children in Need to highlight the help they both had from Hope House when Alfie had to have chemotherapy and his lower left leg amputated after being diagnosed with bone cancer.
The 14-year old spent lockdown learning to walk again with his prosthetic leg – and get back on his bike since his diagnosis in 2018 and almost two years of chemotherapy.
Last weekend the siblings, pupils at Thomas Adams School, featured on BBC Countryfile Extra walking in Corbett Woods with Matt Baker.
Alfie told Matt: "Without the people at Hope House this would have been a lot more difficult. They are like a second family."
Sister Elizabeth, 12, said: "They know what you’re going through and they understand. I also got to meet people who were going through the same thing and it made me feel like I’m not alone."
Their Lake District Triathlon will be broadcast on Countryfile at the end of the month.
The sibling's grandmother, Lynne Jones, who joined them in a Hope House fundraiser walk last year, said watching the programme filmed at Corbett Wood and Grinshill was extremely emotional.
"They showed photos of Alfie when he was ill and it brought back how difficult it was for him, for Elizabeth and the family.
"They both spoke incredibly well about the help they had from Hope House and Matt made them feel totally at ease.
"Alfie is fantastic now and they are both looking forward to the Lake District challenge."
They will be accompanied on their Lake District adventure by their father, Dan Rixon.