Help convincing my mom and dad...?

so far the itunes app store u need to have a credit card in order to download free apps and my parents think if they put there credit card in they think theyll get charged after " the itunes trial ends up" i told em it isnt a trial and the only way to get cahrged is to buy something, and i told them itunes is safe and secure but they wont listen!! Help me convince them, amd im writting this on my ipod touch so sorry for mispelled words