I am reading a bedtime story in Russian to the kids. They are interested, they are following, they seem to understand. And, all of a sudden, my eight-year-old fluent Russian-English bilingual asks, in Russian, “Что значит ‘материя’?” (“What does ‘fabric’ mean?”). This happens a couple more times while I am reading. Some of the words she does not know may also be new to children of her age in Russia. As for other of those words, kids in Russia would be surprised to learn that someone their age does not know such words. This is normal. And this is why I keep reading in Russian to my kids, and insist that they should read in Russian on their own.
This also happens during reading in English. This is normal. And this is why reading is English is necessary too.
We master the sounds and the grammar of our language in the first few years of life, but we keep learning new words throughout our childhood and even throughout our whole life. I recently added a few dozens of new words to my mental lexicon when I started learning to sail. New words are learned with new experiences, with new concepts. Bilingual children tend to have smaller vocabularies in each language than monolingual children. This is because they divide their time between languages. They have different experiences in each language. They might know, for example, science terminology only in English, but names of the plants in their backyard, only in Russian.
But whatever experience they have, books have an enormous potential to expand it. Books contain a lot of words that we rarely use in everyday life – and yet a lot of them are words that every educated speaker knows. The more limited the child’s experience with a language, the more important it is to read books in that language.