You might want to apply for a legal order for the child. This is a good idea if you want to make the arrangement secure legally and if you want to have parental responsibility for the child.
If you do not have parental responsibility you would have to refer back to the parents (or if they are still looked after to the social worker) about all major decisions concerning their care, like medical treatment, signing school trip forms etc.
You may well want to get parental responsibility if the child is going to stay with you a long time.
A legal order such as a Special Guardianship Order or a Residence Order would give you parental responsibility. That would mean you had the legal right to raise the child, and make some important decisions about the child. You could apply for one of these legal orders before the child comes to stay with you, or after the child has been living with you for a while. For more information about these orders look here.