I thought this was good! Very interesting take on the prompt with an interesting twist at the end. All in all it was a fun read! I’ll try to offer some suggestions though.
First of all, I found it hard to read at first because all of your paragraphs are mashed together. There are paragraphs, but you should indent or add an additional line break so that it’s easier to tell where one paragraph ends and another starts. This just break things up and makes it more visually appealing.
Your grammar was good, but there were some small mistakes that could be cleaned up. The most consistent one I noticed was dialogue punctuation; however, this might simply be a difference between “American” English writing rules and “British” English writing rules. I’m from America, and I believe you are not from some other things in your text (like the use of “mum” rather than “mom”). Either way, I’m familiar with only using a comma to attach the speech tag to the quote once. Here is an example:
“Hi,” he said. “My name is bob.”
Vs
“Hi,” he said, “my name is bob.”
This is very minor, of course.
Another thing that you could work on is reducing adverbs by using stronger verbs. This just means that rather than “moved quickly” you say “sprinted”. The one word “sprinted” has the same meaning as the two “moves quickly”. An example from your text might be something like the following:
“Jesus, mum. No!” he said, fleeting sustain in his voice”
Vs
“Jesus, mum. No!” he hissed
Or
“Jesus, mum. No!” he growled
Or
“Jesus, mum. No!” he snarled
You get the idea. This can save you words and make your writing more effective at the same time.
Honestly, that’s about all I can suggest. It was a very good story! I hope to read more clever stories from you!