It's safer to be around strangers than relatives in every study's sampling, because most people don't leave their kids with strangers. They spend most of their time with relatives, friends and acquaintances. If they did spend as much time with strangers as with relatives, you'd conclude it's safer to be around relatives.
It's like saying cyanide is safe because most people don't die from cyanide. Yeah, because most people don't take cyanide.
Children spend waaaay more time around parents and relatives.
Same reason why you are more likely to be killed by an intimate partner than a stranger. (This is not monocausal, and I do not mean to imply that simply spending time with someone will cause a crime)
It really depends on the exact framing of the question.
Are children safer ALONE with a random stranger or with a family member? Why is the child alone with a random stranger?
There are some broad daylight kidnappings, but they are relatively rare. They are relatively rare because it's much easier for a kidnapper to abscond with someone who trusts them.
I'm not sure of the statistics, but even so, it's something you have more control over than a lightning strike.
Edit: at first glance I'm not sure the numbers are right...
Google says: "Lightning damage in the U.S. [...] In 2021, there were a total of 11 fatalities and 69 injuries reported due to lighting in the United States."
Also: "In the United States, an estimated 460,000 children are reported missing every year. Federal Bureau of Investigation, NCIC."
Edit again to be more specific: Many of the "goes missing" doesn't mean kidnaps, and according to wikipedia, "The vast majority of child abduction cases in the United States are parental kidnapping".
However it also says: "Fewer than 350 people under the age of 21 have been abducted by strangers in the United States per year, on average, between 2010–2017."
This number is still a high multiple of the number of people (not just children) struck by lightning each year.
My uninformed guess is that a not insignificant percentage of "children reported missing" are teenagers who run away or stay out past the time they were supposed to return home.
One must remember that about 11% of children are eligible for a drivers license.
So we shouldn't allow children to go outside where the lightning could get them! (/s, but I wouldn't be surprised if someone seriously believes it is what we should do..)
I know a guy who continually insisted he was doing the right thing telling his child to get out of the pool if there was thunder. You know what? Far more people die from accidental pool electrocutions due to bad wiring than lighting (yes, I know this may be due to people getting out of the pool and avoiding death).
It's safer to be around strangers than relatives, when it comes to kidnapping.