If you are ready for your own flock - or have one you can add to - incubating eggs is something every child should have a chance to do. If you don't have a local ranch supply place, Ebay has an amazing variety of fertile eggs you can order (just check on legal restrictions for your area). I have ordered everything from chickens to ducks to quail, from different vendors, and had great success.
Google "how to candle eggs" and you'll come up with dozens of sets of instructions with better pictures than I can take (especially since I don't actually have any eggs to candle right now.) Let the kids number the eggs and keep track of how they look every few days in a science journal or on a chart.
If you don't have anything similar nearby, then - books to the rescue again! Here are a couple we have at our library:
Written in the style of the traditional harvest song, tells the story of a family that toils together to make the work of harvesting lighter.