Refusing to exchange the damaged shoe

I bought a kiddie's sandal at Mr Price beginning of September in 3 weeks time the button from holding the strap came out now they don't wanna take the shoe becaimuse I don't have a slip. I can't find the slip anywhere ànd if the wasn't a problem with the shoe I wouldn't return it. They are telling me about their policy that I can't understand because that's their problem please help me