This is such a broad question. There are too many factors that can affect learning in children. For instance, some children can be hyperactive and attention deficit. If this is so, then these children will have a very hard time staying still and learning. They may even have to take medicine. Children can also have various types of other difficulties. So, the first step is to identify the weakness in a child's learning. Only after this can we develop a plan of action to teach any subject - in this case science.
On the other end, some children may be very accelerated in their learning. In these situations, the material might have to be made more advanced.
With these points having been stated, teachers should always make learning, appropriate to the level of the student, interesting, and applicable. If teachers do these basic points, then the success of learning increases.