I second joining 4h. If you are interested in breeding and I showing i also suggest You have your family join the ARBA http://www.arba.net.
And the ontario rabbit breeders club.
They do not need to be registered, or full pedigreed. Its best that they are purebred, and they meet the breed standard. If your rabbits don't meet, then you can always breed to improve you lines. Don't get discouraged when you go to your first show, and they don't do wel. Always treat showing as a learning experience. you will run into a lot of wonderful people in the rabbit show world that should be more then willing to help you out.
As for feeding, I would suggest keeping the main course to pellets and hay. And feed them fruits and veggies as treats if you want to. They are not nutritionally necessary, and if you want to show, your best bet may be sticking to a diet that is suitable for a show rabbit. otherwise you may end up running into condition and health problems. They may not do well on the table because of it. with show rabbits, you really want to pay close attention to what you are feeding them. You want something that is very high in fiber,and low in protein.
I used to raise Flemish at one time. They are pigs. They ate about a cup to a cup and a half of pellets a day. I never fed mine veggies or greens and they still did just fine.
Since you are new to Flemish, one thing i want to point out is that they are a breed that can have problems with their hocks because of how heavy they can get. You might want to keep an eye on that. Esp if have them on wire.
my other suggestion is to go to a show, and talk to the flemish giant breeders. Its always a good idea to hook up with someone that is more then willing to mentor, and help you out. IMHO that is the best way to learn about showing rabbits.
Dandelions are also fine as treats.