A rocket made of paper, a film canister, water and an antacid tablet is an easy at-home project and also safe enough for kids. Eye protection is recommended for this experiment.
An antacid-powered rocket requires a film canister with a lid that snaps into the inside of the rim. Some photography stores may have spare canisters for free. There are also online stores, like the Science Bob Store, that sell empty canisters specifically for making rockets. Use regular printer paper to make the canister look like a rocket. One piece of paper is enough to yield a nose cone, body and fins. There is no right or wrong way to build the rocket body and experimentation with different shapes and lengths is fun and educational. The paper body should, however, fit over the canister in such a way that the opening of the canister faces away from the nosecone.
Powering and firing the rocket requires a bit of speed and coordination, and adults may need to help very young children. Anyone firing the rocket or standing nearby should don eye protection. With the rocket held upside down fill the canister with water, leaving a bit of air space at the top. Drop the antacid into the container and quickly clamp down the lid, giving the rocket a few shakes. Set the rocket in a clear area and blast-off should occur within seconds as the gases released by the fizzing antacid create enough pressure so the film canister lid pops.