Large Language Models (LLMs) are rapidly becoming part of tools used by children; however, existing benchmarks fail to capture how these models manage language, reasoning, and safety needs that are specific to various ages. We present ChildSafe, a benchmark that evaluates LLM safety through simulated child agents that embody four developmental stages. These agents, grounded in developmental psychology, enable a systematic study of child safety without the ethical implications of involving real children. ChildSafe assesses responses across nine safety dimensions (including privacy, misinformation, and emotional support) using age-weighted scoring in both sensitive and neutral contexts. Multi-turn experiments with multiple LLMs uncover consistent vulnerabilities that vary by simulated age, exposing shortcomings in existing alignment practices. By releasing agent templates, evaluation protocols, and an experimental corpus, we provide a reproducible framework for age-aware safety research. We encourage the community to expand this work with real child-centered data and studies, advancing the development of LLMs that are genuinely safe and developmentally aligned.