We were talking about breathing and diaphragms in Sonlight C science, and we made this really fun model on how our diaphragm works. You'll need to cut a 2 liter bottle in half, cover the bottom with a plastic bag and tape up all the holes with masking tape, and put a balloon at the bottle opening, pushing it into the bottle. Then get some packing tape and make a tab at the bottom of the plastic bag to pull on.
When expiring (breathing out), the diaphragm curves up and pushes air out. You can see the "lung" deflated
When inspiring (breathing ing), the diaphragm flattens to allow space
for lungs to expand. Here you can see the "lung" inflated.