I have no idea whether non-custodial parents are primarily to blame -- these stats cited by ABC News just say family members or parents.

"203,900 kids were abducted in 1999 by family members or parents. Approximately, 58,200 were 'non-family' abductions — only 115 were defined as the frightening kidnappings by strangers." -- http://abcnews.go.com/US/story?id=91365&page=1

Unless almost all of the family cases are non-custodial parents, your claim of hundreds of thousands seems a bit exaggerated. Some of these must be custodial parents, uncles, grandparents, siblings, and so on.

I think 'non-family' means acquaintances, minus the 115 cases of abduction by a stranger.