This is a really good (if morbid) question, AV, and has set me to thinking as well...
I actually have legal custody of our (my and my DH's) two children, because when they were born, DH was going through a hellish custody battle and divorce with HIS ex and I wasn't sure whether DH and I were going to survive that battle (it took almost 4 years, all told). So I went to court in the US (he didn't contest or anything) so that I could get custody and also get court sanctioned child support. However, now that we are married, and living in another country, does my custody still hold up? I guess I should check...if something were to happen to me now, I don't want his custody rights to be questioned.
Sorry to hijack, AV...but you got me thinking!