He has since encouraged his football team, Peterborough United pre-academy, to join him, and their coaches have backed him completely.
Harrison's mum, Racheal Smith, 31, said: "He came up with the idea he would like to do some fundraising for our local hospital to try and get the Amazon ward some new toys and resources they may need.
"His first thought was 'What if they have had to throw items away because a child with coronavirus has played with it for everyone’s safety?' He said I want to make children’s stay in hospital a bit more happier because some children are very poorly and that makes him sad.
"On Thursday, May 7, they will be starting a 5k at 7:30pm in Hampton just in time to clap at 8pm for the keyworkers, hopefully they will be on their last few laps. This will also be recorded on Facebook live for all donators to watch.
"The boys have a designed T-shirt on their way as well to show support for the NHS and their pledges printed on the reverse along with their names."
The football team have pledged to do the following daily:
200 sit ups
200 star jumps
200 press ups
Meanwhile, they will be running 10k per week for two months. The pledge started on Monday, May 4 and the children involved vary in age from six to 11 years old.
To donate, click here.