We usually go to Fresh52 at Tivoli on Saturdays, although I believe they also do one in Henderson on Sundays and Town Square on Fridays. As far as Farmers Markets go in Vegas, this is a good one. Decent mix of fresh fruits and veggies (although unlike many other states, you pay more for fresh fruits and veggies here than at a grocery store), specialty food items, and handmade items.
My ABSOLUTE favorite is 86'd eats. The hummus is the best thing I have ever tasted! Hubby and the kids like JoJo's jerky, and last but not least is TrueFoods, awesome Salsa, and their guacamolito is to die for!
All in all, it is a pretty fun family day, not cheap :-) but tasty!