You would have to use a cold dye, as a hot washing machine dye needs to be boiled in, so you would end up with the heat of the water wrecking the filling of the sleeping bags. Also, if the bags are made of nylon/man made material mix they won't take a dye very well. If they are cotton, you would be able to cold dye them.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.