Browsing the Internet, I've found a few anecdotes which support this view. I don't have any verification that they're true, so you'll just have to take my (and their) word for it.
First story: a young woman is waiting at a street corner. She sees a mother, who is not paying attention, whose child wanders out into the street in front of an oncoming bus. The young woman jumps out and pulls the child back onto the sidewalk. Her reward? The mother yelling "how dare you touch my kid!!" and our would-be hero is treated as a villain, and forced to flee the scene.
Second story: a young man is on the beach. He observes a small male child falling off his surf board a long distance from land. The young man swims out and rescues the child from drowning. On returning to shore, he's greeted by an irate mother who calls the police and wants to press charges for child molestation. Luckily, witnesses confirm the man's story and the cops let him go.
Following this second anecdote a (self-proclaimed) lawyer comments, describing how this situation could have led directly to the young man being registered as a sex offender. By the time police would have questioned the child, his head would be full of misinformation from the angry mom, causing him to tell the police what they "want to hear", possibly putting the Samaritan behind bars or at least requiring a costly and life-disruptive legal defense.
Now, I'm not blaming either the moms in this situation (they are probably freaked out and will naturally accuse the first person they see who might be responsible for their child's endangerment) or the harsh treatment of sex offenders (children should obviously be protected from predators). But it's worth noting the incentive effects that these sort of stories have on potential Good Samaritans.
My personal stance is to never interact with a stranger's child no matter what the circumstances are. I won't engage in conversation, nod, smile, or hold a door open. I was about to say that the most proactive thing I'd do if I saw a child in danger would be to record the incident on video to give to
Being a Good Samaritan is really a lose-lose proposition. If I succeeded in saving the child, best-case scenario I get a pat on the back, worst-case is a sex-crimes trial that will haunt me for the rest of my life. If I fail to save the child (it still falls under the bus) then maybe I get accused of murder or assault because the angry parent saw me "push" the kid instead of trying to rescue it!
There is absolutely no upside to helping or interacting with a stranger's kid. Perversely, this fact makes being a Good Samaritan far worse: because rational people know it's a bad idea to help a kid, the people who do try to help are even more likely to be creeps or labeled as such (the selection effect).
In its efforts to prevent strangers from harming vulnerable children, society has also unintentionally deterred strangers from assisting vulnerable children. It's hard to say which impact is more important, but given the relative magnitudes (there are lots more healthy, well-intentioned people out there than sex offenders) it's very possible the overall effect has been negative for child safety.