You need to prepare your children as you would with a birth child for another child coming into the family. Using books can be helpful (see reading list).
Talk to your child and be honest with them, ask them how they feel and if they have any worries.
Involve your child throughout the process and try to address any issues they may raise. The social worker who will be completing your assessment will also talk to your child and involve them in the process in an age appropriate way. They will be on hand to answer any questions your child might have.