Flora

here's the first description of the little girl, Flora.

The little girl who accompanied Mrs. Grose appeared to me on the spot a creature so charming as to make it a great fortune to have to do with her. She was the most beautiful child I had ever seen, and I afterward wondered that my employer had not told me more of her. I slept little that night

This particular description followed just after the description of the house being so beautiful. It definitely seems as if the governess is simply being blown away by all that she has seen. With the little girl's description/appearance, she adds on to the feeling that all this is otherworldly, almost too good to be true. She is the little doll she is holding in her hand.
She is first described as a "creature" as if her beauty goes so far beyond normality that she can not even begin to be compared to other humans. She also comments on her surprise that her employer has not talked more frequently, or rather, bragged more frequently about the girl. It's as if for the governess this girl is such a gift that the whole world should know of her perfections.
This also goes along with what we've learned of the governess so far. Again she says it was "on the spot" as if she is so caught up with what's going on she doesn't stop to wonder about it all. The perfect horror hero would have already been suspicious, but this governess is not, rather she is the classic naive female who never suspects deception until it occurs.
With this opener passage of the girl, depending if as a reader, one is generally hero/naive. Anyway if the later, like the governess, you will be very caught up in the description of the girl and house, and will not be able to wait and see what the rest of the story leads to.
If the former, then you will automatically know that you should be watching the little girl for anything suspicious. Like the old saying goes if it sounds too good to be true it probably is.