Crete has plenty of gorges to go and see the beautiful nature of the island. Samaria Gorge is big and challenging. Mili Gorge is smaller and easier to see. HopOn HopOff buss from Rethymno can take you there. It is easy to go there just to walk a half kilometer and visit the tavern and have a small adventure with your children. You do need proper walking ability to go there, but you can see quite beautiful scenes just with a short walk (with long stairs).
If you are curious to see more you can walk all the way down and back. That makes for about 6 km all together. But the way back is -of course- up hill. You can also walk only down hill. At the end there's a village and you can call a taxi there to drive you back to your accommodation place; if you want to avoid walking back to up hill.
In Mili Gorge you can pay attention to its flora and fauna and discover the magic of it.