1. Account numbers and passwords for any and all online brokerage accounts.
2. Parents who have minor children must express their wishes, should both parents die, of who should become their children's guardians. Just imagine the grief if the courts need to decide this or if there are relatives who you do not want to have raise your children in the event of your death.
3. In the event of death of both parents, there should be a third party, probably an attorney, who has all of information noted on this thread, i.e, brokerage accounts, passwords, location of will etc.