The answer to that is "But maybe the parents are misinformed about the tooth fairies' abilities?" You can go on and on like this, but at this point I would stop praisuing the child for pursuing the ratinal method for solving problems, and strat educatting the child in the next lesson of rationality: 0 and 1 are not probabilities, all knowledge is probibalistic, and you need to do VoI calculations before rushing off to try to rule out narrow and increasingly unlikly options.