It's a difficult question because the "best way" depends a lot on the child -- their interests, temperment, etc.
As mentioned, setting a good example and praying for him/her are very important.
The more he/she is immersed in the world -- TV, movies, music, sports (viewing, not playing) -- the more difficult it will be for them to develop any sort of appreciation for spiritual things.
I'd recommend a few age-appropriate books, but so many children these days would never open a book they weren't required to read for school.
If he/she has any "normal" interests (nature, history, music, ...) there could be an opportunity to interject elements of the Faith into discussions or activities. On the other hand, if their only interest is in the latest movies, you won't find much opportunity for this.
Especially for younger children, participating in the family rosary is more about building a habit and setting an example than about focusing on the prayers. But, it might help school-aged children to stay attentive by providing pictures to go along with the mysteries or by taking a bit of time to explain each mystery before it is prayed. Of course, this can be helpful at any age, but I find my preschoolers will say the Rosary with us just so they can stay up a few extra minutes and not have to go to bed. : )