Some inventions ideas for kids include a homemade stethoscope and an ocean in a bottle. Some other ideas are a balloon-powered toy car, a water xylophone and a container for eggs that keeps them from breaking if they fall.
It's possible to create a stethoscope using a cardboard paper towel tube, a funnel, a balloon and some duct tape. Insert the funnel into one end of the paper towel tube and tape it with duct tape all the way up the tube to prevent the tube from collapsing. Leave the end of the tube open so the sound of the heart can easily travel through it. Pull the balloon over the other end of the funnel and secure it with tape. Listen to the heart by placing the funnel end of the tube over a person's heart and the child's ear at the other end.
Create an ocean in a bottle using an empty and clear plastic bottle, a funnel, tap water, blue food coloring, and baby or mineral oil. Some other materials to consider using are small shells, glitter or little plastic sea creatures. Strip the bottle of all labels and fill it halfway with tap water. Add blue food coloring through the funnel a few drops at a time, followed by the oil until the bottle is full. Add any other optional materials, such as glitter or seashells, cap the bottle tightly, and shake it to reveal churning blue waves.