The Hoff will be dressed as Captain Hook from Peter Pan when he appears on the show, aiming to raise cash for Pengeulan Primary School in Wales.
He said: "I just hope I can win some big bucks for them.
"One minute I'm terrified, the next I can't wait to jump in to the seat.
"I've been gearing up and I'm ready to face the million pound question."
Welch will be dressed as the Fairy Godmother from Cinderella when she sits beside him.
Also facing question master Chris Tarrant will be comedian Tim Vine, who is on a team with former EastEnders star Cheryl Fergison, who played Heather Trott.
Another regular from Albert Square, John Partridge, will partner TV presenter Alison Hammond.
The Who Wants To Be A Millionaire? Pantomime Special is being screened by ITV1 on Sunday, December 9 at 6.40pm.