I haven't seen the book you're talking about but just because it's a children's book doesn't mean it's going to be easy.
My only advice is grammar, grammar, grammar. I mean actually reading grammar texts from cover to cover. It's dreadfully dull but if you don't get a grip on it you won't progress.
If you're reading something don't worry about finishing it. Take it sentence by sentence and break each one down grammatically so you really see what's going on. For me, language learning is incredibly time consuming and there are no shortcuts at all.
Sounds like you're in school so you probably don't have any choice but reading something without a translation handy is a waste of time.