This is a really complicated situation, you must immediately consult a good attorney and explain him the current situation. You can file a petition in the court challenging the custody and provide the evidence you mentioned. If you have any proof of the complaint which you made to the police, school and the social worker then include that also as it will be helpful in building a tight case against the mother. My friend's sister also faced a similar situation, so she consulted a renowned lawyer over here in Paris and he guided her perfectly.