In her speech she told the audience: "I know how absolutely thrilled Michael would have been to be here to receive such an amazing, significant prize.
"At the end of the day knowing his book is enjoyed by children makes it so worthwhile. He'll be extremely honoured."
When asked what it's like to be the granddaughter of an award winning children's author, Lea told Newsround: "It's no different to a normal granddaughter relationship except for doing things like this!"
Private Peaceful is a very emotional story set in the trenches of World War One.
Rachel Hunter, nine, from Lewes, liked the way the book talks about a period in time not many other children's books do.
She said: "It makes me think how things like that would never happen now."