Am I responsible for a personal debt incurred by my husband to his father before we were married?

I am a lawyer in CT and practice in this area of the law.  I doubt the loan is enforceable as the statute of limitations to enforce the loan has long expired (the statute of limitations to enforce a breach of a written contract is 6 years and 3 years for an oral contract).  Therefore, i do not see a way for the kids the obtain any money from you on this basis regardless if the debt is even valid.

Time really has nothing to do with this case.  The proceeds of both a life insurance policy and a 401K pass directly to the designated beneficiary.  That means that they pass outside of the deceased's probate estate.  Consequently, the beneficiary is in no way legally obligated for payment on any debts of the estate.