Do I think these stunts are childish? Sure. Do I think they're ineffective? That's harder to say--clearly most people disagree with their tactics (and I specifically disagree with blocking traffic because of potential impact to emergency vehicles), but it's gotten a whole tranche of new people talking about climate change, even if it's just to gawk at the weirdos throwing soup.
There is a potential global catastrophe looming. We're capable of solving it if we can just move past our apathy. These folks are doing everything they can to shock us out of our hypnotic trance.