Impossible to make specific recommendations, since everyone works their own way. At best, a few observations.

Formal studio portraitists tend to choose focal lengths between 85mm and 135mm for head-shots or upper body shots. In digital terms, that is about 55mm to 85mm. Your 50mm lens is equivalent to 75mm, so it would be close enough.

If you are shooting children and babies indoors, if it is relatively fast, it means that you can shoot with the 50mm without flash and not hurt the eyes of the little ones. It also allows a bit of working distance, so you are not crowding them.

Outdoor portraits and action shots would probably be most comfortable using the kit lens, giving you a range of focal lengths to cope with a wide range of shooting conditions. If it is the 18mm->70mm, it is a bit slow for indoors - f-3.5 to f-4.5 - but entirely adequate for daylight shooting.

Almost all the portraiture I do is environmental. My goal is to tell the person's story by connecting subject to the location and significant elements in that person's life. For this, I use super-wide lenses. The 18mm is my personal "normal" lens. For decades, I shot with a roll-film Brooks VeriWide 100 - 47mm SuperAngulon over a 6x10 cm frame. Digitally, I use a Nikon CP8400 with a .75x lens component, also giving me the equivalent of 18mm. Were I shooting with a D200, I would want a 12mm prime lens, which I do not believe exists. There is an F-4.0 12mm->24mm zoom, which probably would suffice.