Ironically, this happens because the medicine is working. FTA:

> When a person’s depression starts to lift, he or she may feel less hopeless and helpless. That sounds like an improvement, but when people feel less helpless but still feel depressed, they may think about suicide as a way out, whereas before they were too immobilized to make a suicide plan. For that reason, a decrease in the symptoms of depression can increase the risk of suicidal thoughts or actions.

Not arguing either way, as I don't think I am knowledgeable enough on the subject to do so, but I feel like there is a lot of potential to confuse correlation and causation. It simply could just be that "children and adolescents taking antidepressants" are already in a high risk group that is 3 times as likely "to have suicidal thoughts or to attempt suicide", and antidepressants reduce that number to "twice as likely".