Takealot not wanting to stick with warranty still in effect

In December I shopped around for a quality school bag for my son, as I also am a regular supporter of takealot where possible I went through there bags compared it to prices of the outlets that sells school uniforms, I decided to go with Takealot as I have NEVER been let down with any of my goods, and my son's school bag came with a 6 month warranty. Now they collected his bag Friday the 12th of May, I started enquiring why I have not heard anything back as it was supposed to be a simple exchange, just bring a new bag (as he is now using his old bag again) they came back to me stating that they were declining the return, basically that my son had damaged the bag. This was on the 16th of May, again I had to go bag and say that I didn't want an exchange based on the bag, the wheels didn't even last 5 months! So either process a return so I can purchase the same one that he had before or Bring a replacement bag, as he sure doesn't sit and say let me break the wheels, they have a lot of books which have to go from one class to the next, you can't expect kids to put these bags on their backs. My son is in grade 7 and of small build.