When my son was younger, he really loved taking part in projects that made him feel he was part of the world of science.
I still have fond memories of our time spent collecting rock samples, sending our names into space, and classifying galaxies. There are even more citizen science programs around these days, so if you think your kids might also enjoy this, you can take a closer look.
observe and report weather data
collect and send in a local rock sample and receive a certificate of participation
search through signals for signs of extraterrestrial life with the seti project
document wildlife
classify astronomical features
Finally, here is a searchable database loaded with citizen science programs in many different categories:
Happy Science!