How about something that doesn't involve electronic technology? You might be able to teach 4 year olds how to fold paper airplanes (maybe printing off some pages with fold lines so they can easily follow along). For the 7 year olds, teach them how to play Yahtzee.
Paper airplanes are a rich medium. It's fast and fun to get results, and quick to iterate on improvements. You can start to fold in physics/aerodynamic lessons and introduce them (without a lecture) on how to think like an engineer when you add weight to the front, or flaps to the wings, experiment with number of folds, etc.
