Reddit reviews What Do You Believe?: Big Questions About Religion

Depending on the age of the kid, these could be some good ones:

What Do You Believe?

See Inside World Religions

The Belief Book and others in that series

What is God? (this one does presuppose some kind of a higher power, but compares its depiction across many religions and suggests that god is in us/everywhere around us/basically the concept of good)

I Wonder (not specifically about religion, but more about the idea that science can answer some questions but not yet all - some things are a mystery that we can only wonder about)

Also definitely recommend including some books on the scientific version of the creation story (i.e. the big bang, evolution, etc) so kids have that perspective to draw on as well. Again depending on the age, some good ones are:

Older Than the Stars

Big Bang: The Tongue-Tickling Tale of a Speck That Became Spectacular

One Day a Dot: The Story of You, The Universe, and Everything

Our Family Tree: An Evolution Story

Grandmother Fish

Sapiens: Our Human Evolution

Hope some of those are helpful! :)

Yea /u/Stutturdreki linked to r/atheistparents and I found a book that would be ideal...

https://www.amazon.com/What-You-Believe-Big-Questions/dp/146544386X

Any religion that believes in Christ is Christian which is all of the ones you've listed.
The differences?

Catholic: One of the earliest christian religions. Split with Orthodox Christians about 1000 years ago. Catholics one pope Orthodox Multi-popes.

I'm being very vague BTW

Protestant: Didn't like pope went their own way.

Anglican: Henry the 8th didn't like pope or wife. Got rid of both.

Lutheran: Martin Luther says Nein to Pope! Does it his way.

Jehovah's Witness: Figures all the other Christians are interpreting bible wrong. Puts their own spin on it.

Mormonism: Not only wrong interpretation, they added some more to the bible. Made a Zane Gray Testament of sorts.

Baptist: Again no Pope puts their own spin on the bible. They like water.

There are some great books that give an overview of each religion.
There's a kids book called 'What I believe'. It's actually really good for people who are curious.

I'm an atheist as well but I lean towards Buddhist Philosophy. (which has no deity BTW)
It's great that you're curious. To me a true Atheist harbours no anger towards religion but a curiosity for better understanding.

Edit: Maybe this is the book I was thinking of. You may feel silly but it's a great introduction that isn't too heavy!

https://www.amazon.com/What-You-Believe-Big-Questions/dp/146544386X/ref=pd_sim_14_3?ie=UTF8&psc=1&refRID=BQP09TX3F857CF2R7CEQ