I have personally read through the Bible many times in my life – as you have likely done as well, however; I recently came across a verse which I'm sure I must have read before, but just truly "noticed" . . . .
This is just one example, but no matter the age of the children, grandparents and parents need to be absolutely sure they hear about God, His Word, His great love for them, His amazing grace and how He answers prayer, so they will know Him. They absolutely need to see how their grandparents and parents respect and honor God and experience both talking with God and seeing how He answers their prayers, because at the end of the day, we all want them to grow to be people who know, respect and honor God.
(And, if you are looking for resources to help the families in your ministry with this - check out Science, the Bible & Fun and Generations Quest!)