The best you can do is educate them.
I wrote an entire ESSAY trying to convince my parents to get a snake and answered all their questions and they realized how harmless they were along with the fact they wouldn't have to interact with it and said yes. (The only downside is I have to pay for it all... I guess that's fair though, kinda hard to make money when you're 13 but I am DETERMINED.)
So even if you educate them about all their concerns, yet they say no you gotta respect that.