My parents want my iTunes password and I don't want to give it to them.
Is there a reason why you don't want to share it with your parents?
They have a tendency to charge my credit card high amounts.
It is good to keep your passwords private. Never give out any passwords if possible.
You need to realize that this is a dick move on my parent's part and that I shouldn't be the one to worsen our relationship. They should know it's inappropriate to request the password from me and now I am going to be the bad guy.
It's unfortunate that your parents put you in this position, but I believe you should be firm and let them know exactly why you are denying their request. Setting boundaries is necessary and it's definitely not always easy.