How would you feel if some weird random strangers set up a free cookie hut outside the elementary school? Any kid can get as much free candy and cookies as they want as long as they go inside and don’t tell any adults.

I would say if the companies providing the service do so knowing it is harmful and cover that up then yes they can sue.

> can they knowingly allow their child to do something harmful and then sue because it turned out poorly

That likely depends on how that "something" was publicly marketed to both parents and children based on the company's available information. Our laws historically regulate substances (and their delivery mechanisms) which may lead to addition or are very easy to misuse in a way which leads to permanent harm (see: virtually all mind-altering substances); even nicotine gum is age-restricted like tobacco products. Because nicotine is generally considered an addictive substance, it's regulated, but few reasonable people would argue that parents should be allowed to buy their children nicotine gum so their kids calm down.

Consider how, decades ago, the tobacco companies were implicated in suppressing research demonstrating that tobacco products are harmful to human health. The key here will be if ByteDance has done the same thing.

Also, to play off your point on cheeseburgers: remember the nutritional quality of one cheeseburger versus another will vary. If made with top-quality ingredients (minimally-processed ingredients, organic vegetables, grass-fed beef, etc.), a cheeseburger is actually quite nutritious. However, in a hypothetical situation where a fast-food chain was making false public claims about the composition of their cheeseburgers (e.g., lying about gluten-free buns or organic ingredient status), and someone is harmed as a consequence, the victim might have standing to sue the fast food chain.

I think we would all agree that parents bear a lot of responsibility here. Also, if I think if we look at how we treat kids in other parts of society it's very clear it's a good thing when highly addictive things are kept away from them. It's a good thing cigarette companies can't advertise to children. It's a good thing serving children alcohol or allowing them to buy weed is illegal. And now that we have this new poison, the law hasn't quite caught up yet, but this is a poison, and it's being fed to children with a ferocity and sophistication that only modern technology can provide. A kid can't make a hamburger in their bedroom. They can sneak a phone in and use it. I think it's both. I shout from the roof tops to every parent who will listen to not buy their kid a smart phone. I also think we should hold companies accountable when the knowingly get children addicted to poison.