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.
It's safer to be around strangers than relatives, when it comes to kidnapping.