Good concept with poor execution **SPOILER ALERT**
They could have done so much with this concept everything fell flat. First the kids were made to be too stupid. Your average ten year olds would not so dumb as to be willing to test out a bulletproof vest for fun at least 10 year old I know. First constantly staring down the barrel. The fact that they seemed clueless how to fire a gun and yet at one point Travis racks a shotgun with no issue. With all the gun exposure kids get today in video games not to mention the gun danger talks kids always get today you would think they have a clue. And also the rural region they live in the movie seems like a very gun friendly area so they likely would have had gun exposure by 10.
Their ability to figure out how to drive seemed a bit off considering they have been riding in cars for 10 years and playing photorealistic video games. I mean not to know to put it in gear to get it move. They also didn't seem to concerned that someone would see them on the road.
And the film played like two movies that halfway thru come together. One about kids running away and finding and stealing a cop car. And another about a murderous sheriff. We are never given an explanation as to why the sheriff intended on killing the guys he had in the trunk in the first place. Its clear he is a cold hearted killer since he allows the woman to get shot and die in the road. At the end little Travis getting shot by the ricocheting bullet seems pointless to the story arc. Would have preferred to see him shot intentionally in a situation where he actually tries to protect himself and his friend from either the sheriff or the good/bad guy. The fact that he gets hurts at the point of escape seemed, well, pointless. It didn't improve the story. We are not even sure if the sheriff dies from hitting the cow.
If the writers where going to include little kids in a harsh murder/thriller throw the kids into the thick of it rather than playing it safe. Give them some smarts. Have them running for their lives. Show them forming a closer bond with each other in the process. Have them in real honest to goodness danger. Have them ultimately stand up to the bad guys and the sheriff and have Travis take the bullet protecting his friend. Then have emotional scene where his friend comforts him while he dies in his arms. Yes its sappy and sad but would have added some emotional connection to the characters.