Friends are generally brothers and sisters, and children are younger than themselves. Regardless of their age, they can call their children nephews or nieces. If it's a casual occasion, just call it by its first name.
There are many honorifics in ancient times, such as "your son", "childe", "young master", "Jia er" and "Lin er". Nowadays, there are fewer titles for calling each other's sons, and the common one is "good nephew". If you are a younger child, you can call it XX child and XX baby, which means that you are close to your child. If you are a teenager, you can call them boys, little brothers and so on.