What it might do is stop children killing themselves as a result of adversity.
Rates of death by suicide in the UK have been declining recently, but not for young people. We also know that rates of self-harm for young people are increasing, and self harm is a significant predictor of death by suicide.
Assuming suicides are usually a result of violence (bullying by peers, pressure by parents and teachers...). Wouldn't it be better to show practical ways out of it? Saying "I'm so great and thankful" while bullies pounce on you might feel rather hollow quickly?
Don't get me wrong, this seems to be set up as an experiment, which seems like a good thing. Maybe they'll find it will actually help.
Your argument sounds like "something has to be done, because..." - but something is not always the right thing.
Isn't that basically the job of children, and has always been?
Not to denounce the efforts to improve their mental health. Just saying some struggle seems normal and unavoidable, probably even necessary.