I have been teaching 4th grade for 8 years now, and I have two experiments I do to help understand these concepts. As they often get confused between mass being how much matter is packed in and volume how much space something takes up.
The first one: I take two balloons and blow them up to about the same size. I hang them on two ends of a ruler and pop one. Then the ruler moves like a balance and we begin talking about how air takes up space and how much mass is in the full and empty balloon.
The second one:
material: pan balances, corn flakes, raisins, plastic cups
We fill a cup with corn flakes, one with raisins, and one with a mixture. We talk about how they are all same volume because each cup is a 7 oz. cup. Then we compare their masses in a pan balance. This has always seemed to help them understand the difference.
Good luck with it.