What would be the specific harm of TikTok? Is it because of China, or because of this style of social media apps?
If it's because of China, then ban TikTok and subsidize a competitor app - but be serious about it. Be ready to spend hundreds of millions of dollars on keeping the app free and servers up for years, sponsoring hundreds of content creators, dragging celebrities in to produce exclusive content, burning tons of money on all kinds of loss-generating marketing bullshit, like "publish a video from $concert, best 100 videos win a Nintendo Switch", for each concert in the state/country for the next few years.
Point being, if you can provide a TikTok alternative and make it extremely attractive, even if by literally giving away hundreds of dollars to each new user, then kids won't care about using TikTok anymore.
Now, if it's not about China but teenage use of social media?
Put an absurdly high tax on smartphones. Like 200%-1000%. That will make sure they become unavailable to a typical teenager.
Beyond that? I don't think you can do much about it directly, or in the immediate term.