Something very weird. Trust me, this item will turn up in your travels. These may include sheriff's badges, plastic swords, tiaras, etc. If serendipity doesn't provide these, go to a display and costume shop for these low cost items. Be careful not to let the gender of the children in question define what you put in the box.
Girls like to be lumberjacks and boys like to wear silky stuff too.
Being who you aren't is what costumes are all about. You can give a rating to this scheme overall. Click 0-10 below and press the Submit button. DoBe.org or click on the image below.