If child's dad is the narcissist and they are his parents then chances are they will always take 'his side'. They're also likely cut from the same toxic cloth as he is.
Set boundaries for YOURSELF as in, the treatment you will tolerate. If someone doesn't treat you right, remove them from your life as much as possible.
As pp above says, you have to get comfortable with the idea that some people won't like you.
Bullies for example, never like their victims. Even when they play nice, it's to trick you into a false sense of security. So dont try to be their friend. It won't keep you safe. Just avoid them as much as possible and use the grey rock technique so that you a. Seem like a boring victim. And b. Don't give them any extra amo to use against you.
Never tell them your dreams, goals or desires. If there's something you really want or look forwards to, they will likely try to ruin it for you. Always remember, they see you as the enemy. They don't want good things for you.
Similarly, do not tell them your weaknesses. They will kick you when you are down.