You could do Cyanotypes, and tone them in coffee - you'll get dark brown prints, very gothic, and cyanotypes only need water, Potassium Ferricyanide, Citric Acid and Ferric Ammonium Oxalate, all of which are reasonably safe to handle in the home. Do you want your children to eat them in bulk? No, but normal exposure amounts will be harmless. You contact print cyanotypes, so they only require enough space in the sink to have a tray full of water the size of the final print, and a window in which to expose the prints. Cyanotypes are often marketed as "sun print" kits to teach kids about photography, to give you an idea. Might actually be a good opportunity to share an activity with your kids, unless they're too small to handle a paintbrush with any accuracy.