Even children understand what it means to have no friends.
Literal
Friends [subject-が] not-exist [quotative-と] say thing [subject-が] what-kind thing [question-か] [topic-は], children [even-でさえ] know.
でさえ ('even') is more emphatic than でも — it stresses that children, the least expected group, still understand this. The embedded clause 友達がいないということがどんなことか nominalizes 'what kind of thing it is to have no friends' into the topic.