A few pointers for taking kids fly fishing that work for me.
1) When do you take a child fly fishing? When they ask to go. There is no magic age when they are ready.
2) Where should I go? Go to a place where the odds are very high of catching fish. (ponds are a great idea)
3) What should I bring? Only the childs fishing rod, flies and any tools you would need on a trip. It's a good idea to have drinks and snacks.
4) How long do we fish? When the child says it's time to leave you need to go. It doesn't matter that the fish are biting. You want this to be fun for them. Once it stops being fun or they get hot or whatever it is time to go.
5) Patience and be calm.