My feeling is that they wouldn't have handed themselves in.
Why would they, if they knew they'd lose Victoria that very second? The whole endeavour would have been pointless.
I think they would have kept running, waited for the publicity to die down, and then re-emerged with false identities, or have left the country. Plenty of people go missing and are never found. Often the assumption is that they're dead, but they occasionally turn up alive. John Darwin (of canoe fame) is perhaps one of the more famous; he even got a passport in a false name (and who's to know what happened to Lord Lucan!). They were estranged from everyone, who was to notice they were even missing? As with the case of Margaret Fleming, it can be a long time before anyone notices.
Then, perhaps, in their fantasy, re-emerge years down the line with a healthy, happy Victoria and a younger sibling or two, thus showing the world that they were right all along. At last! Evidence that the problem all along was SS and CM's family.
But no one was ever meant to know that Victoria existed; the car fire is the only reason we ever knew, and it was a sliding doors moment.
If they'd lived as home schooling, anti vaxxer parents whose child remained robustly healthy and never needed medical treatment, would anyone ever have noticed that the child didn't have a birth certificate? How often are parents actually asked for their birth certificate? (Genuine question, I'm not a parent, so I don't know).
Of course it's a fantasy, and the reality is that history would have repeated itself from their time with baby FF, GG and HH, but it's a fantasy I could see them concocting in their heads.
MOO