Plastic lunch boxes, unless otherwise stated, should be cleaned with warm water and dish soap. Some can be cleaned in a dishwasher, but the packaging or website in which that lunch box comes from must explicitly state that. Fabric lunch boxes are a bit trickier. Typically, the interior of fabric lunch boxes is a cleanable plastic, but it’s the exterior that needs special attention. To avoid staining, follow the special instructions given with the lunch box. If you cannot find special cleaning instructions, check the manufacturer's website.