Incorrect labelling

Im very upset as I totally spoiled my 6yr olds Christmas. Bought him a pair of nike takkies 2weeks back. Paid R800, for a size 12. On Christmas morning he struggles to put his shoes on. me thinking its a leather shoe and that it will get more comfortable as he wears it. We get from church and the child immediately removes is shoes saying that it hurts him. and there he is stranded on xmas day with slippers and inhouse the whole day as all his friends/family are bragging with there new shoes. I took the shoes yesterday to check whats wrong maybe the laces needs to be loosened or something. then I check the inside labels of the shoes and discover its a size 10. Then it all makes sense, Edgars shoes hangs on these hangers with the sized on them, and I took the size 12 hanger, but it was a size 10 shoe on the hanger. So who's fault is this now. Im going to return this shoes in this week, and I insist that get the service I deserve as this is not my mistake , but the stores.