I took divorce from my wife and I want custody of my child

My wife cheated on me and ran away. I placed FIR but police didn't help me because they said child is 4 year old and your wife is mature even if we found her you won't able to hold your marriage long time. I've found her through social media and she's posting my child's photos. By looking at photos I think she's hurting my child and child seems to malnourished. I know where she is.. not exact address but city. I'm thinking to contact her but I fear if she abused my child more. I don't know anything and don't know if she's hurting my child but child seems to be malnourished. What can I do get back my child from her legally. What should I do?