2 to 4: Hand you the dog's leash, run a lint roller over the couch (though you'll probably have to go over it again), and feed fish (be sure to keep any electrical wires connected to the tank out of reach).
5 to 9: Hold the middle of the leash (while you hold the handle) during a walk -- as long as you have a well-behaved dog; fill water and food containers for wee animals. Because dogs and cats can get territorial around food, wait until your child is 10 before having him feed Fido or Fluffy. Kids can, however, give pets a treat to reward them for good behavior (but only when you're around).
10 to 12: Help train dogs to obey basic commands, pick up poop in the yard, and assist in cleaning the fish tank. Also, take charge of feeding the animals. But double-check that it got done -- unless you want a very angry cat on your hands!