Different states have different laws. Here's my take on it.
If it were my property, I'd post the whole property. I don't want trespassers on my land. Think of the bigger picture.
What would happen if someone was on your property and got injured? Would your parents be open to a lawsuit by that trespasser? Could they use the argument that "They only posted the area around the pond, so I thought it was O.K. to be on their property!"