For younger kids, you want to keep it simple, yet fun. A little messy doesn't hurt either as long as you are prepared for that. A good project is dripping writing ink onto blotting paper and dripping a splash of water in the center of the ink. The colors used to make up the ink will fan out and separate. Kids are fascinated and can do this virtually by themselves. Projects where they are doing most of the work will keep their attention.
For kids in middle school, getting them interested is the key. Projects that explode are great for this. Using bicarbonate of soda and vinegar to make a water bottle volcano is perfect. It shoots foam out the top. Causing a penny to turn green by setting it in vinegar is another good one. They can do both of these by themselves with very little supervision. Middle-school kids need instant gratification to hold their interest.
High school students is where it will get more complicated. Students this age will be able to follow more complicated instructions and combine chemicals to make something new. A good starter project is making soap. It is not a difficult project, but it does take precision and the results are tangible.
No matter what age the child is, being prepared is essential. Make sure you have the space, safety equipment and all needed tools and chemicals before you start any chemistry project.
Know the ability level of who your working with. Some elementary students can perform at much higher levels, while some high school students may need simpler projects.
You may not need to hover while the project is underway, but always stay available. Sometimes even the simplest of chemistry projects can get out of hand.