Broken mother

I bought two galaxy superboss JL350 20in bicycles for my son's for R1099.00 each on Friday 25 February. On Saturday 26 February one of the bicycle broke the chain. Monday 28 February I took it back to the shop they fixed the chain I could see it jams when the test the chain they said the problem will be fixed. I asked for exchange because am not happy with that one because I could see the problem is not the chain they insisted. I went back home Munson took the bicycle for a ride within less than 5 minutes he was back with a broken chain I called the store to tell them for exchange they told me there is nothing they can do to exchange or refund me is a cheap bicycle. I called the manager he told me the same thing the only thing they can do is change the chain I told them the problem is not the chain it jams and baked the chain they insist on putting the new chain they say no guarantee for bicycles. Yesterdays 28 February the other bike started jamming and taking out the chain. I am so heartbroken with the services of Solomons Pretoria North cycling so disappointed and I will like to have my money back or exchange on working bicycles.