child custody of mother

I badly need advice about the custody of my kids. I'm separated with my husband for 6 years now. The kids are with him for 2 years and 5 months now. We had a written agreement about it. Now i want to get the kids back and transfer them to a much nearer school where i live. But he said we need to revoke the agreement first and i think he doesnt want to give my kids back under my custody. He is living with his mistress with the kids. I know that my kids are having a hard time gettimg along with his partner. What should i do. Will that agreement be a basis that i abandoned my kids and gave him the full custody of my children?what case can i file to get my kids back? Hoping for your immediate response. Thank you.

Palaisipan muna. To establish legal basis:
1. Did you marry? With a valid license?
2. Is your separation recognized by the court? Or physical separation lang?
3. Do the children have birth certificates? NSO registered?
4. Whose surnames do they carry? Tatay? Nanay? With paternal acknowledgement na nasa live birth?
5. What is the nature of the written agreement with the children? Legal?
6. What is your status right now? May kinakasama? Kanino ang anak ng mistress?
7. What are the ages of your children?
After establishing these, assess mo kung may sapat kang suporta, or depending sa situation ng mga legal checkpoints, suggest to lang:
a. pagusapan niyo muna
b. go to court and file a petition to give order on the custody of the children (matagal at mahal. kailangan ng abogado). The court has considerations and follows an order of preference in awarding custody.