Kidnapping was very frequent during the Dark Ages. This was mostly done by foreign invaders who needed kids to work their own lands. Landlords who lacked enough workers, frequently resorted to kidnapping kids in order to populate their own villages. Punishment for these crimes were of a very high magnitude and if the kid was part of the royalty, the offender would be heavily tortured and executed in a public plaza.
Most crimes did occur to merchants. Merchants who traveled alone during the Dark Ages were very prone to such attacks.