Their intent is not what i am criticising, it is how they are going about it. If it were my kid I'd tell them I am proud for wanting to fix the problem, but they are going about it in a wrong and shameful way, harmful to themselves and others without seeing the big picture.
I would encourage them to continue to protest and to be an activist, but to be mindful that their actions and words dont do more harm than good.
More importantly, as an adult, particularly if i were in control of a media outlet. I would refuse to promote or give screen time to the kids, like greta, who are being harmful to the cause and instead give the screen time to the kids who are making real sacrifice and accomplishing good towards the cause. This in turn would encourage kids to take the right sorts of actions to bring about change.