Dark Ride: A Thriller by Lou Berney

Sometimes the person you least expect is just the hero you need
Twenty-one-year-old Hardy “Hardly” Reed—good-natured, easygoing, usually stoned—is drifting through life. A minimum-wage scare actor at an amusement park, he avoids unnecessary effort and unrealistic ambitions.
Then one day he notices two children, around six or seven, sitting all alone on a bench. Hardly checks if they’re okay and sees injuries on both children. Someone is hurting these kids.
He reports the incident to Child Protective Service.
That should be the end of it. After all, Hardly's not even good at looking out for himself so the last thing he wants to do is look out for anyone else. But he's haunted by the two kids, his heart breaking for them. And the more research he does the less he trusts that Child Protective Services —understaffed and overworked—will do anything about it.
That leaves…Hardly. He is probably the last person you’d ever want to count on. But those two kids have nobody else but him. Hardly has to do what's right and help them.
For the first time in his life, Hardly decides to fight for something. This might be the one point in his entire life, he realizes, that is the entire point of his life. He will help those kids.
At first, trying to gather evidence that will force the proper authorities to intervene, Hardly is a total disaster. Gradually, with assistance from unexpected allies, he develops investigative skills and discovers he’s smarter and more capable than he ever imagined.
But Hardly also discovers that the situation is more dangerous than he ever expected. The abusive father who has been hurting these children isn’t just a lawyer—he also runs a violent drug-dealing operation. The mother claims she wants to escape with the kids—but Hardly isn't sure he can trust her.