Quiche and LForida I went in search of some statistics for you. There were several links to find the information you were looking for.
http://www.missingkids.com/missingkids/servlet/PageServlet?LanguageCountry=en_US&PageId=2810
snipped from the above website~
How many children are reported missing each year?
The U.S. Department of Justice reports
* 797,500 children (younger than 18) were reported missing in a one-year period of time studied resulting in an average of 2,185 children being reported missing each day.
* 203,900 children were the victims of family abductions.
* 58,200 children were the victims of non-family abductions.
* 115 children were the victims of stereotypical kidnapping. (These crimes involve someone the child does not know or someone of slight acquaintance, who holds the child overnight, transports the child 50 miles or more, kills the child, demands ransom, or intends to keep the child permanently.)
Other links:
http://www.missingkids.com/en_US/documents/nismart2_nonfamily.pdf
quite a large pamphlet with lots of information at above link.
I also found an article done by abc news. Of course I will leave it up to you to draw your own conclusion but in answer to your specific question of how often an abductor might do so for the child's "own good", it offers quite a compelling argument.
Snip~ Despite what seems to have been a rash of children abducted by strangers this summer, there aren't any more than normal, and vastly greater numbers of children are kidnapped by their own parents and relatives and those cases rarely generate headlines.
Stranger abductions make up the smallest percentage of children reported missing every year. The National Center for Missing and Exploited Children believes that stranger abductions are decreasing overall and projects that there will be approximately 100 of these kind of cases by the end of this year. ~ end snip
http://abcnews.go.com/US/story?id=91365&page=1