Ducklings and Goslings do not usually wander far from their families, sometimes one will stray. If you find one of these stragglers look for their family, then put the baby in the water and it will swim back to them. If you find a nest, leave it alone as the adults will leave the nest to draw away preditors, they will come back after you are gone.
Fawns: The doe will hide her fawn in a field or edge of brush, she then goes off to graze, she has not abandoned the baby, she will be back for it around dusk. Leave the baby where it is and do not touch it. If you are really concerned you could watch with binoculars from about a half a mile away. If mom sees anything threatening near her baby she will not approach. WHY?? Well, fawns do not have an odor and if they stay still a predator will not see or smell them, and don't worry Mom will not forget where she has left them.