If you are honest to them about the fact that you have downloaded Tiktok, then they are less likely to be angry or confused at you about it. They will have more respect for you, because you had the ability to talk to them. If you do not tell your parents about the fact that you have got a TikTok account, and then they later find out, you could be in a lot of trouble. Depending on how strict your parents are, there can be consequences, such as taking your phone away from you. So make sure you talk to your parents about it, because chances are, they will grow to accept it.
Make sure to tell your parents that the main reason you want to join TikTok is to have another way of interacting with your friends. You can also play the ultimate card by stating that your friends are the same age as you, possibly younger, and they have an account. So why shouldn’t you have one?
Tell them that when you have an account on private, it means only people whom you are friends with can see your videos. It may so be important to say that the only way to become friends is if the two users follow each other. Tell them that you will not do anything to risk your safety on the site, such as accept friend requests from any random people.
If it is about security reasons, then you can always reassure them that you will be as safe as you can be on there, and that you believe that you have the knowledge to keep yourself safe online.