Make comments that show you are intrigued. Let yourself see things with a child's eye. Be reflective without being judgmental. Be comfortable with the way your child evaluates their work. Don't expect perfection or hold your child to your adult standard.
Some young inventors are very critical of themselves.
If this sounds familiar, help your child by reminding them that inventions usually don't work on the first trial. Inventors don't see trials as failures, but as steps to success. Many students need help with time management skills. Divide the project into small steps and set timelines together. Help with follow-through.