The increasing prevalence of AI in everyday life has intensified the emphasis on teaching AI literacy to children. However, there is no consensus on the specific knowledge and skills that constitute children's AI literacy, resulting in varied AI learning materials for young people. We systematically searched for educational practices for children's AI learning in both formal and informal settings and examined the AI learning content taught to children. Our findings led to the development of a holistic AI literacy framework for children, which contains three high-level dimensions and eight content areas of AI literacy: AI awareness (AI definition, AI application, and AI history), AI mechanics (AI input, learning procedure, and AI output), and AI impacts (AI implication and responsible practice). Theoretically, we contribute a research-based, comprehensive, and current framework for children's AI literacy, advancing its conceptualization in early life stages. Practically, our framework can guide researchers and practitioners in promoting AI education for the next generation.