Unregistered

When I was little I saw a painting (I believe in the Naitonal Gallery?) of two girls in a doorway or window; the girls look happy, and I think one of them is covering her face with her apron. I thought the title was 'secret' or had something to do with that, but I have looked everywhere, and I cannot find it. If anyone has any idea about the title or artist, it would be much appreciated!