If you wanna create characters from scratch any 3D modelling software could serve you, but i'd recommend Blender since its free has a more active community and there are a lot of addons to speed up the workflow and tutorials. I'm not experienced with Daz Studio which is the software used to create many of the final renders you see in AVN, especially those made with Ren'Py. In Daz, you can "create" some lolis using Growing Up tool (paid one) using an already created adult female. You can also download a ready-made one. Maybe an experienced user of this program could help you more about this.