The warming of our planet matters to every child. Driven by fossil fuel-generated greenhouse gas emissions, climate conditions stable since the founding of modern pediatrics in the mid-nineteenth century have shifted, and old certainties are falling away. Children's physical and mental health are threatened by climate change through its effects on temperature, precipitation, and extreme weather; ecological disruption; and community disruption. These impacts expose and amplify existing inequities and create unprecedented intergenerational injustice. Fossil fuel extraction and combustion cause harm today and reach centuries into the future, jeopardizing the health, safety, and prosperity of today's children and future generations. Appreciating the unique vulnerability of their patients, pediatricians have become leading health advocates for climate actions necessary to protect all living and future children. Policies that reduce reliance on fossil fuels and promote cleaner air, facilitate walking and bicycling, encourage more sustainable diets, increase access to nature, and develop more connected communities lead to immediate gains in child health and equity, and build a foundation for generations of children to thrive.