To address your question, "summarize me this content what this not want to say," let's break it down. The content provided is an open letter to UN Secretary-General Antonio Guterres, focusing on the inclusion of children in the Youth Advisory Group on Climate Change. The main point of the letter is to advocate for the meaningful inclusion of young climate activists, specifically those under 18, in international policy discussions on climate change.

The letter highlights several key points:
Congratulations on the Youth Advisory Group: The authors appreciate the establishment of the Youth Advisory Group on climate change, which includes young leaders between 18 and 28 years old.
Disappointment over exclusion: However, they express disappointment that the group does not include children under 18, who have demonstrated significant activism and contribution to the climate debate.
Children as agents of change: The letter emphasizes the role of children in sparking global movements, such as Greta Thunberg's solo protest, and their ability to influence through schools, social media, and street protests.
Request for inclusion: The authors request that the Youth Advisory Group be inclusive of children and adolescents under 18, respecting their right to be heard and contribute their expertise directly.
Call to action: They also ask for the UK and Italy to provide children with a safe space to influence upcoming COP26 preparatory events and for facilitating meaningful engagement with children globally on climate issues.

Given this context, when you ask what the content does not want to say, we can infer a few things:
It does not want to dismiss the efforts of young people already included in the Youth Advisory Group: The letter acknowledges the importance of the group but seeks to expand its inclusivity.
It does not argue against climate action or the importance of addressing climate change: Instead, it emphasizes the need for more inclusive decision-making processes in tackling the climate crisis.
It does not suggest that children are incapable of contributing meaningfully to climate discussions: On the contrary, it highlights their capabilities and the importance of their perspectives in finding solutions.
It does not propose excluding any current members of the Youth Advisory Group: The request is for expansion and inclusion, not replacement or exclusion.

In essence, the content is advocating for a more inclusive approach to climate decision-making, valuing the contributions of all individuals, regardless of age, in addressing the global climate crisis. What specific aspects of the content or its implications would