You won't necessarily get the rewards from this if your friends complete challenges or levels, so you'll probably want to plan a meetup to actually complete some of the later levels. The next time your friends log on they can continue from where you left off. Once you invite an online player to your zoo, at anytime you can load it up from the cloud and continue your adventures and save it back. Players can gift animals to friends to help breed rare types. There are other co-op options, too that are more subtle. Needless to say the size your zoo can be is impressive, though you won't be able to fill up the landmass before hitting an exhibit limit.
Entering third person mode and jumping into a buggy, I drove its full length.
The game stuttered for a second and then promptly filled in two giant LONG paths connecting everything. Of course, the first thing we did in co-op was try to break the game, so we placed a janitor station (and an antelope exhibit) the furthest we could from the main zoo. It's here where the game shines and the simple act of talking through where to place certain items or deciding what will shape your zoo transcends "business meeting" and usually enters in to the realm of ridiculous. Co-Op allows players to split up duties too - one can concentrate on expansion while another worries about the animal welfare and happiness. It was really impressive to see the giant exhibits being placed by the co-zookeepers while in the third person perspective, though the zoo patrons paid no attention to the giant hovering savannahs. I took to the online play and built out a zoo with a friend both at the same time, and on different nights when we couldn't meet up to play together. For instances the challenges might consist of holding a certain happiness rating for your animals in a set time period while the campaign might just ask you to get your zoo to a certain level. The challenge and campaign modes are similar though have different style of goals. All of the modes of play can be experience cooperatively. If you're looking for a greater challenge Zoo Tycoon extends into a Campaign mode and a challenge mode. In terms of playing with younger kids, Zoo Tycoon offers a "free" mode where you are given unlimited funds and can quickly level up your zoo keeper and see all the animals at your disposal. Up to four people can contribute to a zoo both in realtime and asynchronously. All that said Zoo Tycoon is a great co-op game, whether you're just playing pass the controller co-op on the couch (like I played with my kid), or you are taking advantage of the game's unique online co-op modes.