You could look at this a couple ways: one, she's so tight and callous that if something did happen to the kids, she didn't want to have to shell out any money in the event of their deaths...........or two, she's planning something evil.
But it strikes me funny that she would be asking you for their SSN's when she can get them herself. DS's dad never asked me for DS's, but requested a duplicate card from the SSA. He needed it to open a money market acct. for DS.
I will tell you though....reading your post sent chills up my spine. I just read an article on abcnews.com about a 5 y.o. still missing (I believe in NC) and they've arrested the mother for human trafficking and abuse. The child had been in the custody of the father ALL her life until about a month ago, when the father decided to give the mother the opportunity to raise her, since it 'appeared' that she'd turned her life around. He now regrets it. And there have been way too many child abductions/murders lately across the nation recently.
I would strongly recommend talking to an atty. and maybe your local law enforcement agancy and get their opinion of the situation. Ask them if there is anything you should be concerned about and what, if anything, you should do about it. And let us know what happens with this..........I, for one, am very concerned.