Hm, I think the national security concerns are far heftier here than kids being influenced. ByteDance can be forced to do almost anything by the CCP. Sure, American tech companies might be forced to do things by the American govt but theoretically there are checks / balances / rule of law to prevent misuse.
On the kids' manipulation -- even though that's not the main intent and American social media co's influence kids as well -- I'd still argue it's significant. A foreign adversary can control how your population thinks... it almost speaks for itself.