Daniel Craig is far too old for Frank. He and Helen are meant to be a young couple. 

We haven't had any confirmation of the casting of any familiar characters yet other than Digory and Polly, Digory's mother, and Jadis. Daniel Craig is confirmed but his role hasn't been disclosed. No further word either on whether Meryl Streep really HAS been cast as Aslan, or as some other character, or whether some other actor (preferably male) has been chosen for Aslan, or whatever.

It's still all mostly mysterious and I'm still baffled at how badly Netflix seems to handle their PR, and at how little regard (so far) Gerwig seems to have for being faithful to the book. But we still just don't know enough to be sure about anything much.

That said, regarding Frank and Helen specifically, they aren't "iconic" characters in the original story — we barely learn anything about them as people, and they're pretty much just dragged in (literally) for the purpose of giving Narnia a good, honest, humble pair of salt-of-the-earth working-class people as the new land's first King and Queen. There are several much more memorable characters within the story — apart from Digory and Polly themselves, there's Uncle Andrew, and Jadis (the former is the most likely role for Daniel Craig, and the latter has been confirmed in the casting already), and Fledge as well. And of course Aslan! I would think we'll hear about their casting before we hear about Frank and Helen. But we just don't know. 

Posted by: @courtenay

... at how little regard (so far) Gerwig seems to have for being faithful to the book. 

To be fair, of the very few things we do know about her adaptation, one of the biggest is that she has been exceptionally faithful to the opening line of the book, so I don't think any of the changes really come from a lack of regard.

Anyway, in terms of a prediction...

Option 1 is "never" because they have been cut from the story, which I don't think is true, as I do actually think this movie will be a lot more faithful than people expect right now.

Option 2 is it will be in a press release along with the main cast, which my current best guess would be either at the 1 year mark in November (which is when IMAX did its poster and trailer release for The Odyssey) or around Christmas time once filming has finished. However we've been wrong so many times about when Netfllix will finally play it's hand on a press release, that I'm not holding my breath.

Option 3 would be at Tudum 2026 next summer, assuming that's an annual event, though last year's didn't really strike me as having that sort of vibe for a big reveal like that.

And then I guess Option 4 is when the movie actually releases on Thanksgiving 2026, in the event that Frank and Helen are either such minor actors so as to not be worth prior reporting, or they are such massive stars that they are aiming for a Marvel style surprise cameo.