That's right the law is the law. It's not the kids fault obviously but if you sign something over willingly like your assets of course the other party has a right to claim them.
Perhaps they should check on the authencity of the marriage claim why would a man just simply marry a woman out of the blue in secret?
The man may not have realised that the marriage certificate is effectively a transfer of half his assets to the wife - he should have made a will.