Is this the same people that were behind the Polar Express at Weardale?
If so, they’ve got a good reputation and they know what they’re doing, but the southern WCML is worlds away from any heritage line. Heritage lines will have a (usually rural) station dressed up as as ‘the North Pole’ where people get off to meet Santa as part of the experience.
Going to a freight yard in suburban London where the passengers will have to stay on the train simply won’t have the same feel to it no matter how good the actors onboard are.
The boarding process is also a key part of the experience, Heritage railways can throw all of their resources into making it a good experience. There’ll be people in costume to greet passengers the second they enter the station and the whole thing will be an experience from the moment they step into the station to the moment they leave.
Euston simply can’t match that or indeed come anywhere close. Even if they do manage to avoid the last minute platform announcement and rush, they’ll have to confine the ‘experience’ to the platform they’re using and the train. The passengers will have to navigate the crowded concourse at Euston before they meet a single person in costume and again as soon as they’re off the train they’ll have to navigate the crowded concourse.
Might not seem like big issues but when you add them all together, there’s simply no comparison between the type of operation a heritage line can do for this and anything possible from any London terminal.
On the other hand the traction doesn’t really matter. The passengers will only see the buffer stops end of the train at Euston, so simply have something Steam there even if its not actually powering and stick whatever is convenient on the other end, the kids won’t see it so it doesn’t matter if it’s a scruffy 57 powering the train