This is a good, sharp short, with nice resonances (shark/loan shark and so on) and a good sort of character arc, but it underplays the dramatic moments and the contrast between the imaginative world of the child and the bleak reality of the adults, which should be central to the drama. Kerry's plight needs to be much more played up. Vince needs to be more distinctive and less generic as an antagonist, as at present he is a pretty cardboard cut-out gangster kind of figure. Give him something to individuate him!
The trouble with the tone of the film is that when Kerry goes out on the game, it comes as something of a jarring shift in tone. Unless we are prepared in the previous scenes (between Vince and Kerry, and her anxiousness as she leaves her son alone) for how dark it's going to get, this scene might seem a bit of a stretch.
The last scene is somewhat disjointed, and unless it is intimated at least that the son does at some level understand what is happening and that the shark is in some way a manifestation of his desire to defend his mother, a reaction against his powerlessness, then it all seems a bit lacking in cause-and-effexct, a bit of a deus ex machina. Again Vince's dialogue here is generic, and him asking 'what are you drawing' makes the whole drawing the shark into the scene seem a bit fortuitous.
Overall though overhauling Vince a bit and clarifying the tone and contrasts of the script should result in a tidy little short!