Include another item to compare to the pinecones and acorns, such as leaves. Children are often amazed that something in a small pile (such as leaves or acorns) can have the same number of items as something in a big pile (such as the pinecones). Experiences like this will help them grasp this concept.
This concept might be too difficult for young children. Instead, collect acorns at your local park and count them together. You can help your child “keep track” while counting the acorns by removing an acorn once it is counted. This will prevent counting in circles.