Two young girls in bathing suits laugh while standing under a fountain of water.
Two young girls in bathing suits are standing under a fountain of water at an amusement park with their family.
Sentence 1 does not say where the girls are or if they are with their family.
One can stand under a fountain without their family, and fountain is not seen in amusement park only.
Just because girls are under a fountain of water does not mean she is at an amusement park with their family.