Why did the convict choose Pip to complete the task?

Abel Magwitch is an escaped convict on the run after breaking out of a prison ship. He's roaming around the Romney Marshes like a frightened animal—cold, hungry, and petrified at being caught. He comes across Pip in the graveyard, there to pay his respects to his late parents. After making sure there's no one else around, Magwitch grabs hold of Pip and tells him to go off and get him some food, drink, and some tools from the smithy to help him cut off his leg irons and chains. If Pip doesn't do as Magwitch tells him, he threatens to cut out his heart and liver and eat them. Frightened out of his wits, Pip does as he is told.
Magwitch doesn't have much choice in getting Pip to perform this task. He's so desperate that he has to trust someone. And Magwitch probably figures that it's better to get an impressionable child to do his bidding, instead of an adult who's most likely to run off and raise the alarm at the earliest opportunity. Pip is just a small boy, still naive in the ways of the world. That means he can easily be scared and intimidated by Magwitch into fetching those items he needs for his survival.