Lest you think this is typical turn-of-the-century female modesty or some very poor entrants in a hide and seek competition, I shall explain. Or let the Museum explain anyway:
'The first photographic images in the late 1820s had to be exposed for hours in order to capture them on film. Improvements in the technology led to this exposure time being drastically cut down to minutes, then seconds, throughout the 19th century. But in the meantime, the long exposures gave us a few unmistakable Victorian photography conventions, such as the stiff postures and unsmiling faces of people trying to remain perfectly still while their photograph was being taken.'
'Seems children were just as squirmy then as they are today, because another amusing convention developed: photographs containing hidden mothers trying to keep their little ones still enough for a non-blurry picture.'
You can see more of them here at the Museum. They've also been featured on Chris Wild's always excellent Retronaut site and there's a great archive at the Hidden Mothers Flickr Group. Do visit them.