Children in World War Two

Hi! I'm researching about the children in world war two. I understand that they need to evacuate because of the bombings and air raids. But I read that the children sent their letters to their families and vice versa.

So here's the question:

Does the children and the family's decision to make a letter or the government would tell them?

Another question:

But what if the child's family is gone. Does their foster family can automatically adopt them or do they have to ask permission to the government?

If they have to ask permission, what is the processes to adopt the child?