store assistants refusing a customer exchange of goods

I bought a stress ball for 5 year daughter,whom i was with in the store(Mr price clothing wonderpark mall) and in less than 4 minutes after we had left the store,the ball broke and released some liquid which was inside. We went back to the store to try and see if we can exchange it as it broke few minutes after leaving the store. To my suprise,they refused to exchange it and said my daughter broke it because she wanted to see what was inside,of which was not true and the other excuse was that my daughter started using whilst we were still shopping in the store. i felt it was unfair especially since it broke after leaving the store.and felt maybe it was factory fault related