To expect the benefits of bilingual education parents should support the habit of learning in a second language from the early childhood.
For preschoolers it can be books of their interest or new games. I try to read English books as often as my children wish to (which is almost daily). It is really worth it to have some nice titles at stock at home. No textbook will ever be so interesting as Winnie the Pooh, Peppa Pig stories, The Lion King or Cinderella with beautiful illustrations.
Do you also get questions from kids such as: “What happend to dinosaurs?” or “What is the glass made of?” We explore the answers also with the help of English videos on YouTube such as from Planet Earth from the BBC where you can find many answers.
For school children I recommend Khan Academy www.khanacademy.org We are using their practical videos for example to practice mathematics in more interesting way.
Children should feel that another language is useful for them and brings new knowledge.