We added on a petting zoo when the economy first started crumbling during The Great Recession. I figured that many parents would struggle to take all their kids on expensive trips, especially given the demographic of where we live. So a nice petty zoo with a modest entry fee was just the thing for a fun family day out. We educated the kids about the animals and had a gift store too. It couldn't last forever though. The economy began to improve after a few years, and by then, the kids had seen what we had a few times over. Interest wanes over time unless you keep adding on, which we didn't want to do. It was a profitable endeavor and well worth it though.
I don't have any experience, but it sounds like a very cool idea! What animals do you have on the farm that the kids could interact with? I think it would be cool to have a cow or a pig tame enough to let kids touch it since these aren't often included in your traditional petting zoos.