This isn't about kids or their health, or even about information gathering (though those certainly make things worse). It's about widespread manipulation of the population and about national security. It's about letting an adversarial government have the ability to influence your population through a direct communication channel, whether it's 6-year-olds or 60-year-olds. Hurting or helping children's mental health or collecting information on a population is not a prerequisite for manipulating people, showing them propaganda, or otherwise influencing them in a dangerous way.