My Ex Wife Kidnapped My Daughter

 My ex and mother of my triplets and I are separated. The children were with me.

Last week she called me to inform me that she is in the village and she wants to come over to see the children. I told her no problem my mother is at home with the kids she can come over anytime and she came on Monday while I was a t work. She brought foodstuffs and toys for the kids. 

She texted me to asked if she can take them to her mother's place I said yes. She left with the 3 children. I expected her to bring them back today she said she is going back to Lagos tomorrow and this evening I came home I met my boys, not my girl. My mother said it was her brother who dropped them and I saw a big luggage with plenty of clothes she bought for them. I asked my mum what did the brother said about my girl he said my ex told her to inform me that she will bring her back tomorrow morning. I called, her number is switched off so I went at her mother's place, she said she left with the girl to Lagos yesterday. 

I am very upset that she took the girl to Lagos without my consent and I want my girl back. 

What should I do to have my child back with me?