I don't think B says "it won't become usual", just describes the state this kid is in. How is 普通の子共じゃない different from it? I looked it up in dictionaries, but I still can't figure out the rule when I should use naru instead of a copula.(A and B discuss what they have to do with a very peculiar-looking infant they found and who literally hatched out from a sort of an egg)
▲ここから下り坂になる. It is downhill from here.
･平野の周辺は丘になっている. Hills edge the plain.
･この先は三叉路(さんさろ)になっている. There is a three-forked road ahead.
･地下へ下りるとそこは駐車場になっていた. Down on the underground floors there is a parking lot.