Let me get this out of the way first; no rescue missions! Kidnapped princesses are such a genre cliche that I can barely write this sentence without waves of nausea. There will be no rescue missions in this post, I want the PCs out there grabbing up innocent children in the name of a greater good.
Lawful parties may find themselves morally torn at the idea of grabbing an innocent to forward their cause. This is because they are modern people with modern attitudes.
Twentieth century kidnappings are brutal affairs. Usually a gang grabs the child of a wealthy family and demands money. Often, body parts are used to emphasize the point that non-payment is a poor decision. Many times, the money changes hands and the victim still dies. The “Lindberg Kidnapping ” being the classic example. The most worrying thing is the uncertainty. The kidnappers are dishonorable scum and will kill without hesitation, regardless if they get the ransom or not.
Roll back the clock to the Middle Ages and “ransom” becomes a far more civilized affair. Victims were fighting men from great houses captured alive on the field of battle. A captive could expect good treatment, in fact many were guests in the houses of their captors! Once the ransom arrived, the “guest” was sent on his way with not a hair harmed on his head. Primarily because captors often fielded their own knights and wanted them to receive the same treatment. It was quite civilized and profitable.
Unless you run an “evil” campaign, a modern kidnapping is out of the question. However, a “polite” kidnapping is a great plot hook. Grabbing up a knight after a battle is fun, but stealing into an enemy castle and liberating a daughter is even more fun. See my post on “heists ” for ideas. There is no question that she will not be harmed in any way, but simply having her is great leverage .
Of course, the kidnapping, escape and negotiation with the victim’s family offer great opportunities for some combats and role-playing.
Morally dubious as this is, throw it at your PCs and watch them revel in doing some dirty tricks for a worthy cause.