It's been a long time ago since I've been reading bilingual books. Since I was a 6-year-old child, to be more precise, and... believe me, that it has proved to be really good for my language development.
My mother tongue is Spanish, but I've started learning English as a child. My father used to buy fairy tales or fabules which they brought a casette (lol a casette people, that WAS a long time ago). So, while I was reading I listened to the narration on the casette. Believe it or not, I was incredibly hooked by this kind of books.
As I told you before, these books were bilingual. That means, one side of the casette contained the Spanish version (my mother tongue), whereas the other side had the English version. Wasn't it awesome? For me it WAS, because once I've listened to the spanish version I listened to the english one. But... Where was the text in english to follow? you may wonder... It was in the bottom part of the page, in lower case.
Today, I know that there are books which bring two versions, that is, one page in english and on the other side page the spanish version (or other languages). If I had to recommend these books, I would highly suggest parents buying at least one of them, because they expose kids to two languages at the same time. Pictures also help them distinguish among meanings and analogies, which you may know, it's not easy work. Nowadays, market offers books with cds, containing both language versions (as casettes used to be). So, getting one of these books for kids, would be a nice way of introducing abother language to them. They'll discover they can read the same story they already know in their mother tongue, in another language. And that, in the long run will be really profitable.