It's quite tricky to explain it. In my opionion, semantically, '니까' emphasizes predicative parts(verbs or 'be adj', NOT objects) Thinking childhood is not an inevitable result from 'reading', but it can result from the contents or pictures in the book So, '책을 보니까 어렸을 때를 생각했어요' sounds 'BECAUSE of the 'action' that I 'read' the book, I thought my childhood' '책을 보니까 울었어요' sounds 'Because of the action that I read the book, I cried'. Not because of 'the book' It's okay to say '배가 고프니까 밥을 먹는다'(Because of the fact that I 'am hungry', I eat) '책을 보니까 졸리다' (Because of the 'action' of reading the book, I feel sleepy) '책을 보니까 엄마가 나를 칭찬한다' (Because of the 'action' that I read the book, mom pays me a compliment) (I'm afraid of making you even more confused...) Anyway, it is okay to say '책을 보고 어렸을 때를 생각했어요'. (I read the book, and I thought my childhood)

Well... if you just say 어렸을 때를 생각 했어요 It is correct. But if you say that something makes you recall or come up with something Then we say 생각이 났어요. It sounds like you didn't intend to recall it but you did cuz of something. For the same reason, you can also say 떠올랐어요