Two little blond girls are sitting on a white chair .
Two girls sit on a white chair
Two little blond girls are sitting on a white chair.
Two girls sit on a white chair
Two little blond girls is just a more detailed description of the two girls and sitting on a white chair is the same as sit on a white chair, just a different tense.
Two little blond girls is just a more detailed description of the two girls and sitting on a white chair is the same as sit on a white chair , just a different tense .