Parent, Grandparent, King, and Travel passwords are encoded with the username. Unless you're willing to reset your Tamagotchi then set its username to someone elses username, or happen to find someone who chose the same username that you did, their passwords will not work for you.
If you just want to see what the inside of the Parent & Grandparent's houses look like, you can use my passwords. In order for them to work, you have to log onto TamaTown using my username.
username: TMGC!
password to see parent: 94619 80890
password to see grandparent: 44667 45775
Remember, since you're not using your own username, the passwords for the souvenirs that the parent & grandparent gives you will not work on your Tamagotchi.