Am I misreading? The op doesn't seem to me to be saying she is remarrying. She asked about the case where you leave assets to your ac and the ac predeceases the cil who then remarries, and is outlived by their new spouse.
Nanny, are you concerned that this may happen with your ac and cil? If so, I believe you can write your will so that it says if your ac predeceases you (God forbid), their inheritance will then go to their children/your gc. But if, sadly, you're gone and your ac has already inherited the assets? What happens to them after that? Idk. That might be up to ac and cil to decide. They can probably set up trusts, etc. that would go directly to the gc and a new partner couldn't get control of. But that's up to them, surely.
But I may be wrong. Best to ask a solicitor, Imo.