Some NPCs cant be targetted (e.g. the kids in saint denis, gang members, etc.) - is there a way do circumvent that?

use the fight club mod and just fight like micah or someone that ur not supposed to kill and afterwards you should be able to kill alout of unkillable npcs, works for me atleast

You can find all the default information in update/common/data/relationships.meta including all groups and their relationships. Since you set the relationship group per ped, it is definitely worth trying to work with it. We did it for RDRFR and it seems fine for the most part.

you can not kill children in this game how ever im pretty shure there is a mod that lets you kill kids but to kill gang member use the PDO mod and turn on friendly fire or if you cant mod very well get  dynamite or molotavs and throw it next to someone in your gang