Lucas Priestley, four, was born with the condition which means he is unable to walk or talk.
But his family have now launched a campaign to raise money to buy him a specialist bike.
Miss Hayward, 26, said: “Lucas is such a cute child and he has an infectious smile. It’s just such a shame to see him struggle.
“We want to raise up to £3,000 to buy him the bike so that he can play because he gets bored and frustrated when he can’t.”
Miss Hayward will take part in the sponsored marathon on Saturday, October 3 at BODY Carnegie Centre in Bolton-upon-Dearne.
She said: “I just wanted to do something to help him because he is my nephew and to see him and his family struggle is heartbreaking.
“I do Thai boxing anyway and some of his other family and friends have raised money so I thought I’d do this.
“We just want to make him and his mum, Stephanie, and dad, Luke, happy because it can be a really struggle for them all.”
Miss Hayward is hoping to raise most of the £3,000 target through Saturday’s event.
She said: “We are hoping to raise between £2,000 and £3,000 from the Thai boxing event.
“We’ve got other events happening as a family to help Lucas but this is the main one.”
For more information or if you would like to sponsor Miss Hayward or donate to Lucas’ appeal call her on 07490 488891.