Cottontail rabbits, which are born March through October, are often found in the center of yards. The mothers plan it that way because she knows most predators usually don't go into wide open spaces, and her peripheral vision when sitting at a distance from the nest allows her a clear view of anything approaching her babies.
 

Mom rabbit only goes to her nest when she is feeding her babies, usually only twice a day, generally at dusk and dawn and then only for five minutes at a time. Because of this, most people think the mother has abandoned the nest when they find it and never see the mom around. But it's important to be absolutely sure that the babies are truly orphaned before bringing them to any rehabilitator, as  babies often die if taken from their nest.

The best way to check and see if the nest has been visited by mom is to mark the top of it with thin twigs in the mark of a tic-tac-toe pattern and check after 24 HOURS (but not at dusk or dawn when she may be hesitant to nurse if she sees you there). If the twigs have been moved, mom has been there.

Cottontails are born furless with their eyes and ears closed. They are fully furred at one week of age. Their eyes open a few days later. In another week and a half they will wander away from the nest and be on their own. Because baby rabbits are only in the nest for three weeks, it's not unusual to see a tiny one in your yard.

If something other than mom has gotten into the nest (your dog for example), try to rebuild it and check to see if any of the babies have any injuries. If you have found a nest in your yard, accompany your dog outside if you know he's discovered it. Outdoor cats should wear a bell to alert wildlife, but it would be safer for the wild animals - and the cat - for the cat to remain indoors. If you have to mow the lawn, put a laundry basket on top of the nest with a couple of bricks to hold it down until you're done. If your dog needs access to the area near the nest, you can cut two small exits in a laundry basket, place it over the nest, and weigh it down securely for a more long-term solution.

Remember, the best thing you can do to help the cottontails in your yard is to leave the nest alone. Mom will take better care of her babies than the best rehabber ever could.