Makro Springfield park

We had purchased a bike for my sons 7 th birthday in August 2018. In november as he was riding the bike the chain broke. We paid R900 for his gift. I goo back to makro in november and the guy at customer service says sorry he cannot help as the chain is not covered. So we buy the bike and it was supposed to only last 3 months. We paid 900 for this. The child only used it probably 4 times and ever since it's parked of. Is this fair to customers. How do we know the bike was not a defected item??? How much money we spend and this is the service and quality we receive!!!