All three of my kids have fly fished from a young age - although both my oldest (both girls) now have other interest, my son is 13 and has been casting a fly since he was 5 or 6 and is off the chart crazy for the sport and a damn good caster! My best advice would be get low end gear that you aren't concerned about them dropping in the sand or throwing down when a frog hops in front of them! Let them be kids and enjoy nature - including skipping stones when a fish is rising and helping them find bugs and frogs if that is what they want to do for an hour. It will grow on them and in no time they will be focused on casting and fishing with flies - but only when they are ready to enjoy it.
