I started reading Greek mythology with Helen a few months before we visited Crete and the Cyclades, beginning with the D'Aulaires' Book of Greek Myths, which she picked after I read her one story from that and one from the D'Aulaires' Book of Norse Myths. This is a superb book, with lively prose and lovely illustrations. It was too big to travel with, however, so most of her reading on the trip was from an ebook version of the Waterfields' The Greek Myths: Stories of the Greek Gods and Heroes Vividly Retold — this is not a children's version, so needed on-the-fly simplification in places — and her one book acquisition was The Wonder of Crete and Minos, a children's book translated from the Greek. (An alternative to the D'Aulaires' book was the Usborne Greek Myths for Young Children, but that was less fun for me, though it may work better once Helen starts reading for herself.)
We've since moved on to Lucy Coats' Atticus the Storyteller's 100 Greek Myths, which we are also listening to as an audio book. This has a framing story in which Atticus travels around Greece and the Aegean with his donkey Melissa, telling stories. Helen loves this too - and it's written for 4-8 year olds instead of 9-12 year olds. From my early childhood reading I remember only Roger Lanceyln Green's Tales of the Greek Heroes and Rex Warner's Men and Gods (which may still be in one of my unopened boxes).
From my father I inherited copies of Bullfinch's Mythology and Robert Graves' The Greek Myths, which I have recently unpacked — I showed Helen the inscribed "H. Yee" and explained that that would work for her too. And somewhere in as yet unopened boxes are copies of Michael Grant's Myths of the Greeks and Romans and Yves Bonnefoy's Greek and Egyptian Mythologies. My only recent reading in this area is Roberto Calasso's The Marriage of Cadmus and Harmony, which is superb but not much use as a reference.
But I've got interested in this again now. Most of my reading on Greek mythology and religion is from my "pre-historiographical" period, from before I developed any kind of understanding of historical method, source analysis, and so forth. (As a teenager I was philosophically and historiographically rather naive.) So it will be interesting to revisit this whole realm! I'm also trying to give Helen (in small doses, and when it comes naturally) a feel for some of the complexities — an understanding that there are different versions of the myths, that the neatly connected narratives I'm reading to her are pieced together from different sources and reworked, and that the stories served (and serve) political and explanatory purposes.
I've started with Barbara Graziosi's excellent The Gods of Olympus, looking at how ideas about the Olympian gods changed from archaic Greece through to the Renaissance, and the (now a bit dated, but accessible) Easterling and Muir edited Greek Religion and Society. I plan to follow those up with Peter Parker's On Greek Religion and maybe the Cambridge Companion to Greek Mythology.