Is it okay if I turn down the offer of plane tickets from my parents?
Why are you considering turning them down?
It costs a lot and right now I worry about travel safety.
Your parents might be disappointed but I'm sure they will understand your reasons. Changing your plans will be okay, you can always visit another time.
I should visit them eventually but I have so much to do.
Your parents will understand that you are too busy to travel with them and there will be another time to visit together. Tell them how much you'll miss them and you'll make it up later.