Jack Howland and Matty Walker are running to raise money to help toddler Alyssa, from Coventry.
Tommy Hurrell, 20, is undertaking daily therapy at a rehabilitation facility in London.
Tommy Hurrell says people with "a wide range of disabilities" are offering him support.
The couple say their daughter Ivy, 4, would have loved the special post box to put her drawings in.
There are not enough accessible equipment in parks for children with disabilities, a parent says.
Schoolchildren and staff at St Patrick's College in Dungannon have designed a walking aid for their teacher's daughter, Penny.
Amber McRae has both spina bifida and hydrocephalus, which builds up fluid in her brain.
Alex, who wants to raise awareness of Spina Bifida, is according to doctors, a real-life superhero.
Alfie's parents say the trip, where he also made passenger announcements, will give him confidence.
