You could just declare that your father is an important person and suggest that they’d get more money by ransoming the two of them.
Ransom only works if you can get away with it. basically situation where either you are unknown to both kidnapped person and people looking for them (and have the means to safely retrieve the money before escaping, assuming someone will actually pay you), or have means to enforce your safety (ransoming noble prisoners of war for example).
Here they are probably somewhere in the city (there will be extensive search, house by house if needed), can't escape easily with their merchandise (in this situation any exits would be guarded, any cargo searched).
Once they realize they kidnapped kid of the lord of the area AND kid of one of the military commanders of the area, they basically have to abandon their whole operation and run away.
Kids - either kill them to leave no witnesses (no faces, so potentially easier in the long run - IF they get away), or just leave them alive in place, hoping that the fact that they were found alive and relatively unharmed will mean that they won't be searched with the full power of the forces there (but their faces are still known as the wanted men).
Sure, this dude will probably help them escape and get a pardon for that, but realistically he'd get painless death as a prize at best.