Vicki, 44, from St Martins, has made an emotional video, in conjunction with the hospice, to explain which it means so much to the family.
She will run the 26.2 mile race - postponed by the coronavirus - wearing a superhero cape sporting the names of Poppy and hundreds of other children who have been helped by Hope House at Oswestry and Ty Gobaith in Conwy.
"I had the idea about wearing a superhero cape with the names of children on it because running a marathon is something they'll never be able to do. It will be an honour to fly them around London on my shoulders," said Vicki.
"Poppy is so full of joy and she has taught me so much because she takes life with two hands and she lives it. I know the marathon is going to be hard but when the going gets tough I will think of Poppy and all the other amazing children and that will spur me on to the finish."
Poppy was diagnosed with Dravet syndrome just before her fifth birthday - a rare condition that causes life-threatening seizures. Thankfully, the doctor referred her family to Hope House.
Vicki said: "We took some time to grieve, and counselling at Hope House helped me to accept how things were going to be. We had our bundle of joy and she was still our bundle of joy - it was just different.
"The worst part of Poppy's condition is that it can cause sudden unexplained death. She has had thousands upon thousands of seizures so we can never rest and think that we are safe. Now we try really hard to live each day with as many smiles as possible.
"When we first visited Hope House with Poppy it immediately felt like home. Poppy loved the playroom with all the toys and the nurses were so knowledgeable. The bedrooms weren't at all like hospital rooms and that was really important because we had spent so much time in hospital.
"Our biggest fear was seizures happening during the night, but at Hope House there is always a nurse monitoring her and medical expertise on hand. That is a level of care above what we can even provide at home so it gives us complete peace of mind."
Vicki has set herself the ambitious target of raising £26,200 from her marathon challenge - £1,000 for every mile.
"We've had eight years of great support from Hope House that has meant the world to our family and to Poppy," adds Vicki.
"I'm not sure we would have managed in the same way without our Hope House family and this gives me the opportunity to give something back. The more money I can raise the more care can be given to even more seriously ill children.
Meanwhile Poppy, who is a pupil at the Severndale Academy in Shrewsbury, is very excited about her mum's marathon."
"She is very excited and likes to help me with my training by running around the block with me. It's tricky because too much exertion brings on seizures but we have a little run when we can and she loves to feel involved," adds Vicki.
"When I'm at the start line waiting to go, I'll be thinking of all the children and how every penny people are so generously donating will be put to such fantastic use at Hope House and Ty Gobaith."
You can sponsor Vicki at hopehouse.org.uk/Appeal/poppys-story