It's hard to say, I guess it depends on the kid's maturity and attention span. I think the biggest thing would be tailoring an educational program so it's fun for them.
I'd probably wait another couple years, till they're about six or seven. But at their age now, you could always take them out to a bluegill pond with a handful of poppers and let them tool around for a while...
let them learn the basics with the conventional rod and reel, but start teaching them the cast with the fly rod, then slowly get them to understand what to use in whatever situation given. let them follow you with spinners and once they start to master the cast bring the fly rods along so they can choose what'd they'd like to use that day.
It depends on the kid. But fly fishing requires more coordination, timing, depth perception, etc. than the ole snoopy pole with a bobber and worm. And until kids hit a certain age, as long as their fishin' w/dad, they won't really see any difference between the Winston and Abel and their little snoopy or barbie rig...so long as they catch a sunfish every now and then. You'll know when that changes. They won't want to fish with the spin-casting gear anymore. They'll pout for the fly fishing gear.
When you teach fly fishing to them, keep the teaching sessions very short...10 minutes or so at a stretch. Break the casting process down into it's elements and teach them 1 at a time. Lots of positive reinforcement techniques! Don't teach a whole bunch of these elements and then try and put them together. Teach 2-3 and then put them together, even though you don't have a whole cast yet. Practice that until it is smooth, then go to 1 more individual element, master it, and then add that...and so on. You can turn these drills into GAMES in their own right, so that each day for about 3 days you play a game they can "win" by building the skills you are teaching without actually mastering the whole cast. Progressive and very attainable goals and rewards for achieving them are very important when teaching kids. And the more sophisticated the task, the more important this sort of approach becomes.
I know folks who have successfully taught 4-5 year olds to fly fish, and I've known 10-12 year olds who still weren't coordinated enough to "get it" in spite of high quality instruction.
I don't see why they can't use a fly rod and some line as to dab a line. Give
them some cool looking foam bugs, and they can catch bluegill all day. That counts as flyfishing for me, and they can even use nymohs and indicators.
I use to be into baitcasting, but decided to abandon it completely because
of the damage plug hooks can cause to fish. I use the smallest fly I can get
away with now, and not having to dig a baited hook out of a bluegill could
keep your kids interested in fishing .
I started fishing with the long rod when I was 7, but I didn't really get the hang of fly casting until I was 12. I had a rod that would have given anyone fits however.
Early on, I did get pretty good at what could be called 'high stick nymphing' even though my bait was dug in the garden
Get them out to the pond for gills... Take a shorter flyrod for them to try, but; don't force the issue. You could get some of the graphite Crappie pole and let them dap flies for the gills. The most important thing for kids that age is that they catch something...
Teaching/allowing bad technique >>>> bad habits >>>>> bad muscle memory >>>>> very hard to unteach >>>>> harder to learn the right way later.
I've probably taught about 50 kids to fly fish now, including 2 of my own nephews.
Getting them to catching a few fish early is important, but not at the expense of getting them comfortable with the equipment and technique FIRST. And that should be done in a way that is fundamentally sound, or you're just screwing them up for later.
With kids, I like to use slow action rods that are at least 7.5' to 8' long w/double taper or triangle taper lines and teach them to roll cast first. THEN take them to a pond chock full of pan fish or trout!
The TFO Bug Launcher is a good rod for this. It isn't slow, but it's Medium action and very durable. it has a reduced diameter grip for kid-sized hands and an extended butt so they can use 2 hands to play a fish if needed. I'd probably over-line the Bug Launcher by 1 line weight.
I started my youngest a year ago. He just turned 8 and I wish I had started him earlier. I took a Wally world special, tied some fly line directly to the top eye, no reel, no leader. We started with that in the front yard and did accuracy drills. Out on the creek he didn't have a reel to fiddle with, we added leader and a fly, and he had enough line to get to most anywhere he needed. We practiced stealth tactics, he loved that...