As @Detritus says Terry Pratchett is a good read for a child that is at that level of understanding

You Got To Be Kidding! A Radical Satire of The Bible Kindle Edition
by Joe Wenke (Author)

Not directly anti xtian but the book “small gods” by Pratchett shows the problems of all religions while being pretty funny and interesting. Otherwise “50 reasons people give for believing in god” by Guy P. Harrison will address whatever fallacy he uses to support his belief. A bit dry but comprehensive.

Go for a soft option, not a hard attack. Try. 'The Book of Atheist Spirituality.' by Andre Comte-Sponville. Or even more indirect. 'The Sacred Depths Of Nature.' by Usula Goodenough.

"Why I am not a CHristian" Bertrand Russell

I don't have any titles for you but any book that breaks down the history of its construction as well as the meaning of some of the phrases. I would also say any translations of the oldest versions directly from early aramaic and hebrew would likely also help. The thing is most of what we in the states have in print right now derive from the KJ1 version which was from several other translations and edits as it was. Even standard versions that drop the KJ off it's cover often still come from it.Finding something more directly translated would be one heck of an eye opener to many of them I'd think. I think the real trick would be getting them to actually read anything. Most that set in their faith typically isn't willing to read anything against it.