the narrowest part of the food pipe is the entry to the oesophagus - the criciphaaryngeus. in case the foreign body passes through this it most likely will pass through the entire tract. a round coin would now be covereed by other intestinal particles and would be safe. the danger arises with sharp foreign bodies like pin, hooks etc,not with foreign bodies with rounded edges.
continue to monitor the stools. in case she is asymptomatic, do nothing more. there is no benefit of any one type of food over another, so stay on a normal diet. repeat an x ray after a gap of two weeks or so, soas to let her recover from previous radiation exposure. in case it still persists you will need toconsult a paaediatric gastroenterologist - the chancee is remote.