"In elementary grades, especially sacramental years, we often teach concepts that are somewhat abstract and difficult to understand. How can we do this in a way that children this age will be able to grasp the material?"
According to the well-known developmental theorist Jean Piaget, children are very concrete thinkers in the years between ages 6-7 and 11-12. They have trouble understanding hypothetical scenarios and symbols. In short, they understand what they can see and experience with their senses.
This can indeed be a challenge when we are trying to introduce more abstract concepts or prepare for new experiences. One rule of thumb is to be sure we keep definitions and explanations as clear and concise as possible. Use many visual aids, as well as things children can touch and manipulate with their hands. For example, a lesson on baptism that happens in the church near the baptismal font and ends with the children blessing themselves with holy water might be more effective than simply talking about it in the classroom. Going to visit the Reconciliation Room prior to First Penance might help to ease fears of the unknown. Role-playing new actions or experiences, such as walking through the Eucharistic procession and practicing how to hold one's hands when receiving, can also be helpful.