Living things, earth science, and physical science
Elementary science introduces plants, animals, the human body, weather, space, and materials. More importantly, it introduces a way of working: ask a question, make a prediction, test it, and see what happens.
Children arrive with strong existing ideas about how the world works, and many of them are wrong. Teaching science at this age is largely about surfacing those ideas and testing them, rather than filling an empty space.
Typically living things, materials and their properties, Earth and space, and basic forces, alongside the practical skills of predicting, observing, and recording.
Say you do not know and look it up together. Modelling that is far more valuable than an invented answer, and it shows that not knowing is where science starts.
Because their existing explanations usually fit their everyday experience. Simply being told the correct answer rarely displaces them, which is why testing predictions works better.