In some respects it's not unlike a camping site such as this.  It's a group of like minded youngsters who will often raise an animal or grow produce.  Some of it may be entered into local county fair contests.

If you're familiar with Boys & Girls Clubs, this is similar but rural.

I wasn't in 4H as a kid but instead I was in FFA "future farmers of amerca" I was involved in several things ranging from, learning Parlimentary Procedure, to welding, to equipment repair, to raising livestock & produce, to being active in our chapter as an officer and being involved in judging contests. My DW was involved in 4H and FFA, both clubs run the gammit of learning experiences similar to boy scouts and girl scouts.
I never became a farmer as of a result in my FFA activities nor is my wife a farmer from her experiences with 4H or FFA but I will say that all of the activities we participated in have taught us both many great experiences.
It's good for your kids to get involved with extracuricular activities and to learn from those experiences. I would say check out your local chapters and if the ideals of their teachings are within your expectations let your kids join and learn. You can also be active in the group and share your experiences as well. Everyone has a chance to benefit.