First, I noticed a filming error. When Dexter's going through one of the empty houses, you can see a face in the bottom left corner of a set of jalousie windows. I thought that someone was watching him, but it's part of the film crew.
Now for the mistake Dexter made: he should have taken care of Trinity after hitting him with the shovel. When Trinity was on the ground, Dexter should have injected him with that sodium-whatever-it-is that he keeps in his syringe. Even though the little boy was sinking in the concrete, there was no chance that the concrete would have set up any harder during the time it would have taken Dexter to incapacitate Trinity. Dexter would still be able to rescue the boy and he would have had the opportunity to kill Trinity that same night.
Maybe they had to write that kind of mistake into the show to set up the final two episodes, but I don't think that Dexter's character should have gone for the boy first. Maybe this is part of his changing as a result of having children himself, but the old Dexter would definitely have taken care of business with Trinity first...especially because he knows that a new kill cycle is just being started with the boy, so there are 3 more people in immediate danger. Bad Dexter.