Lambs (sleeping lambs/ones that are laying down tend to be the norm) have been used on the headstones of children for many, many years. I know they are prevalent in the U.S., but I'm not sure about other countries. They symbolize the innocence and youth of the sweet little one that has been lost.
I'm not a parent, but I hope to be someday, and I can't imagine losing a child. Headstones like this always choke me up because I can imagine the parents' grief; the thoughts of what could have been/who this child could have been also come to mind. I know that losing a child was more common a hundred or so years ago, but that couldn't have made it any easier.