Yes..... but it's not SS rules, it's just the law. It's because they both have parental responsibility they were actually back to 50/50 contact at the time, so that played in her favour. It takes months to sort out with the courts and she knows that , so it can be strung out. Most reasonable human beings would never contemplate doing such a thing. Kieran could have kept the kids too when she crashed the car but most reasonable human beings recognise that would start loads of conflict for the kids.
I think she planned the whole thing, after what she was overheard saying at the NTA awards.