I'm still 99% certain that this is just a scene written entirely for the audition, and not something that is going to be included in the actual film when it's made. Rather than lifting a scene directly from the book — which people applying for the audition could have studied and practised beforehand — they've given the applicants a specially written script that they won't have seen before, with two invented characters that are similar to Polly and Digory in their general attitudes, but with totally original dialogue. It's something that would test these young actors in their ability to portray a range of emotions in quick succession, in the sort of scene you might get in a late-Victorian-era children's fantasy story, but not an actual scene (or the actual characters) that will be in the film itself.

Another giveaway to that is the fact that whatever Isadore has in his hand, we're not told it's a ring, or what it is at all. The vague hints given — that it's something he found "on the pavement", that Frannie has "never seen one like this before" and that it's "almost... like you could see another world inside it" — don't really suggest a ring, but perhaps something translucent that you can see into (a glass marble, maybe, or a kind of gemstone?). I get the feeling it's deliberately kept vague because this isn't a scene that's going to be in the real script. It's just something that's been made up to get the right general kind of interaction and tone between these two characters for the audition.

So really, I suspect the assumptions flying around — "They've made Polly an orphan! That won't fit with the story!"; "Digory just picked up the ring from the pavement? What about Uncle Andrew's study and all that?" — are going to turn out to be another big fat nothingburger.