Talking cycles and repetition, this is probably the most prominent example everybody remembers from primary school, the water cycle. The rain comes down over the mountains filters through the stone, a spring, a creek, a river, a stream into the ocean, where it is picked up by clouds, ferries to the mountains.
A fairly romantic picture, but definitely a basic diagram explaining connection. Also it servers very well as a starting point for questions. For example for a six year old, why is the water salty in the ocean, but not the rain? How does the water know the way to the ocean? And why are the couds not drifting of to the moon?
This could go on and on developing into a full blown session on earthly systems, but it is all about the basic idea of a systemic concept. It is not about what happens it is more about what happens with it.
So or an ad explaining what a water supplier company does, this is a gold one, back to school.