In “the girl who silenced the world for five minutes,” what is the most likely reason suzuki speaks about herself as “only a child”?

Aside from the fact that Severn Cullis-Suzuki is actually a 12-13-year-old child at the time of the convention. She speaks about herself as "only a child" because she is helpless of the situation, about the future. That what she can do is to rely on the people that can bring change.
Children's future depends on adults that could see the problems like a child does.