The Strictly favourites will be reunited with their former pro partners Anton Du Beke and Flavia Cacace for a special one-off routine.
Watch a teaser of their Children in Need Strictly performance:
"I'm thrilled to be back on Children in Need as I was in the first one ever! And to be dancing with Flavia once more is a Strictly dream come true," said Grant.
"A fantastic cause doing something I love! Everyone's a winner!"
Ann Widdecombe said: "To reunite with Anton for another Strictly dance is fantastic, but to do a group dance with Russell Grant and Flavia Cacace too is even better. It was a lot of fun to do and we scaled some dizzy heights for the great cause that is BBC Children in Need."
Children in Need airs on BBC One from 7.30pm on Friday (November 16).