Absurd returns policy of bicycle supplier

Early in November 2018 we bought two 20" kids bicycles from a shop in Bronkhorstspruit. The brand was SupaBike. It was Christmas presents and we kept it stored in the back of our closets untill 24 December 2018. My sons are 7 and 9 years old, and only learned to ride bike this holiday. The 9 year old had problems with his bike from the start. The seat kept on lowering, no matter how tight we would make the nuts; the seat kept on going forward/backward, and eventually split in two because we tightened the nuts to much; after only 5 days of riding, the handlebar broke lose from the rest of the frame. At this time we took it back to the seller, and grudgingly they replaced that part for us; another 5 days later the horizontal bar broke where it connects to the from part of the bike - now the bike is in two parts. We contacted the supplier and asked what can be done about this. A person named Ashfaaq Gani rep**** and said we could take the bike back to the seller and they would help us. I asked for a refund because my son now does not want to ride bike anymore - he feels it is his fault that the bike broke. I received a reply again from Ashfaaq, saying that the company's return policy is that the item should be returned in the same original condition in which it was bought. Only if it is determined by them that a factory fault occurred, would a refund be possible. And even then, a 15% handling will be charged.

This is totally absurd!!! How can they refund a product that is made of poor quality. My son does not weight more than 25kg! He did not jump off a building with the bike! He went over a small bump in our yard - on grass- when it just snapped in two. Had he gone faster, he would have gotten seriously hurt. The production of the bike was poor, so how can they now expect me to give them a broken bike back in the same condition? That is the whole reason I asked for a refund, because I have no confidence in the product/brand name anymore!

Please can someone help?