Oh, how do they not know salvation is the best gift ever!
May I suggest, that in addition to giving the truth to your nephews and nieces, they can get a another gift, of salvation by praying to our LORD?
Or could I suggest another gift as well - maybe a poster of hell. Or maybe dropping a red hot coal into their grubby grasping hands, so they can at least appreciate the pain they will suffer by following their secular ungodly ways.
Truly, I believe, until we have suffered such pain, not even a 1/1000000th of the pain of our LORD on the cross, can we appreciate the gift given to us.
Bless the LORD