The Online Etymology Dictionary defines "kidnap"as 1680s, compound of kid "child" and nap "snatch Away," variant of nab... In the book cited, The Old Whig, printed earlier in 1650, Samuel Chandler places the word kidnap in a spiritual context. He castigates the clergy of the day, whom he claims to be "shepherds and so have a right to fold their sheep that the wolves mayn't come near them and devour them." ..."They are to watch over the souls of the Christian people that thieves and robbers may not filch and kidnap them to destruction." p39
The argument here is metaphorical and linked to a number of Bible passages. The kidnapping relates to thinking adults of immature understanding, not to young children.
Kidnap is likely from "kid", a young goat (Hebrew gediy, a young goat, Strongs Concordance,1423) and nap a variant of nab, which Websters (1854) suggests is from KNAB, and means "to seize with the teeth," thus presenting us with a vivid word picture of the seizing of a young member (spiritually) of the flock (a young goat) by the teeth of wolves,(the clergy speaking hypocritically).
"They declare their assent to 39 whole articles only in jest and for financial gain." p398