I assume that you live in England as the law is different in Scotland. You will need to go back to court to get the order overturned. You will be appointed a court appointed social worker, this is from Cafcass, they will prepare a report for court. Due to the children's ages, their views will be taken but not always acted upon.
Your solicitor will be best placed to advise you, it is difficult to return children in these circumstances but be very careful not returning your children as this can go against you in any future court hearing. You say you have serious concerns about the safety of your children, being hit etc, I suggest you again contact social services and seek their advice about returning them and also mention that this is a child protection matter, this may get your children to you in the short term, if they agree.
I'm glad you are involved in your children's life and hope it turns out ok for you
Post again, if you want more info